Hairy Cell Leukemia (HCL): Understanding Your Pathology Report

Section Editor: David Li MD
June 24, 2026

Hairy cell leukemia (HCL) is a rare, slow-growing blood cancer that develops from mature B cells, a type of white blood cell that normally helps the body fight infection. The cancer cells have fine, hair-like projections on their surface that can be seen under the microscope, which is how the disease got its name. Hairy cell leukemia primarily affects the bone marrow, blood, and spleen, and only rarely other organs. As the hairy cells build up, the bone marrow makes fewer normal blood cells, which leads to most of the symptoms.

This article will help you understand the findings in your pathology report for hairy cell leukemia, what each term means, and why it matters for your care.

What causes hairy cell leukemia?

The exact cause of hairy cell leukemia (HCL) is not well understood, but the disease is strongly linked to a specific genetic change. A few factors are relevant:

BRAF V600E mutation — A change in a gene called BRAF is found in more than 95% of cases. This mutation activates a signal that tells cells to keep growing, and it is considered a defining feature of the disease.
Environmental exposures — Contact with certain chemicals, such as pesticides or benzene, has been associated with a higher risk, though no single exposure has been confirmed as a cause.
Family history — Rarely, hairy cell leukemia runs in families, suggesting that inherited factors may play a small role in some cases.

Who gets hairy cell leukemia?

Hairy cell leukemia (HCL) is very rare, making up about 2% of all leukemias. It is about four to five times more common in men than in women, and it is usually diagnosed in adults around 60 years of age. It is very rare in children and young adults, and it is more common in people of European descent than in those of Asian, African, or Arab descent.

What are the symptoms of hairy cell leukemia?

The symptoms of hairy cell leukemia (HCL) develop because the hairy cells crowd the bone marrow and collect in the spleen. Common symptoms include:

Fatigue and weakness — Caused by a shortage of red blood cells (anemia).
Frequent infections — Caused by a shortage of normal infection-fighting white blood cells, including a low level of a cell type called monocytes, which is characteristic of this disease.
Easy bruising or bleeding — Caused by a shortage of platelets.
Discomfort or fullness in the upper-left abdomen — Caused by an enlarged spleen.
Fever, night sweats, or unexplained weight loss.

The liver may also be enlarged. Unlike many other blood cancers, swollen lymph nodes are uncommon in hairy cell leukemia, especially early on.

How is the diagnosis made?

The diagnosis of hairy cell leukemia (HCL) is made by examining the blood and bone marrow. A complete blood count (CBC) often shows low counts of red blood cells, white blood cells, and platelets (a pattern called pancytopenia), along with a characteristically low monocyte count. When a blood smear is examined under the microscope, a pathologist may see abnormal “hairy” cells.

A bone marrow biopsy is usually needed to confirm the diagnosis. In hairy cell leukemia, the marrow often cannot be drawn out as liquid (a result sometimes called a “dry tap”) because the hairy cells cause the marrow to become fibrous, so a small core of solid bone marrow tissue is taken instead. Under the microscope, hairy cells are often described as having a “fried-egg” appearance because each cell has a rounded center with a clear rim of cytoplasm (the material around the nucleus).

Specialized tests confirm the diagnosis. Flow cytometry and immunohistochemistry use antibodies to detect proteins on and inside the cells. Hairy cells carry a distinctive combination of markers, typically CD20, CD22, CD11c, CD25, CD103, CD123, and annexin A1, which together separate hairy cell leukemia from other B-cell cancers. Testing for the BRAF V600E mutation, often by next-generation sequencing (NGS) or a targeted test, supports the diagnosis and is described in more detail below.

What do hairy cells look like under the microscope?

Under the microscope, hairy cell leukemia (HCL) is made up of small to medium B cells with a rounded or oval nucleus and a moderate amount of pale cytoplasm. The most recognizable feature is the thin, hair-like projections that extend from the cell surface, which are easiest to see on a blood smear. In the bone marrow, the cells are often evenly spaced with clear cytoplasm around each nucleus, producing the “fried-egg” pattern that pathologists associate with this disease.

Genetic changes and molecular testing in hairy cell leukemia

Hairy cell leukemia (HCL) is defined by a single, characteristic genetic change. The BRAF V600E mutation is found in more than 95% of cases. This change is important for two reasons. First, it helps confirm the diagnosis and separates classic hairy cell leukemia from a small number of similar-looking conditions that do not carry the mutation. Second, it is a treatment target: drugs called BRAF inhibitors are designed to block the abnormal protein produced by the mutation, and they are used when standard treatment is no longer working. Your report will state whether the BRAF V600E mutation was detected.

What is the prognosis for hairy cell leukemia?

Hairy cell leukemia (HCL) generally has a very good prognosis, particularly with modern treatment. The standard chemotherapy drugs used for this disease often produce long-lasting remission, and many patients have a life expectancy close to normal. Hairy cell leukemia can come back years after treatment, but it usually responds again to further therapy. Testing for small amounts of remaining disease (measurable or minimal residual disease) and regular follow-up help the care team watch for any signs that the disease is returning.

What happens after a diagnosis of hairy cell leukemia?

Once hairy cell leukemia (HCL) is confirmed, the care team plans the next steps based on the blood counts, symptoms, and overall health. The pathology findings help guide several decisions:

Observation — If the blood counts are near normal and there are no symptoms, treatment may not be needed right away. Instead, the disease is watched closely, and treatment begins only when it is required.
First-line chemotherapy — When treatment is needed, a class of drugs called purine analogs (cladribine or pentostatin), sometimes combined with an antibody drug called rituximab, is the main treatment and often produces a long remission.
BRAF-targeted therapy — For disease that comes back or does not respond, BRAF inhibitors (such as vemurafenib or dabrafenib), often combined with rituximab, take advantage of the BRAF V600E mutation in the cancer cells.
Other options — Additional treatments, including other targeted drugs and, in rare cases, removal of the spleen, may be considered for disease that is difficult to control.

Care is usually provided by a hematologist or oncologist, along with other specialists, and blood counts and follow-up tests are used to assess how well treatment is working. Decisions about whether and when to treat are made by the care team together with the patient, based on the specific findings in the report. Clinical trials may also be an option to discuss.

Questions to ask your doctor

Was the BRAF V600E mutation found in my hairy cell leukemia?
What did the flow cytometry show about the markers on the cells?
Do my blood counts show low red blood cells, white blood cells, or platelets?
Is my spleen enlarged, and does that affect my treatment?
Do I need treatment now, or is it reasonable to watch and wait?
If I need treatment, would a purine analog such as cladribine be the first step?
Could rituximab or a BRAF inhibitor be part of my treatment?
How will my response to treatment, including measurable residual disease, be monitored?
What signs should I watch for that might mean the disease is coming back?
What is my long-term outlook based on my specific results?
Are there clinical trials that I should consider?