The diagnosis of HCL may be suggested by abnormal results on a complete blood count (CBC), but additional testing is necessary to confirm the diagnosis. A CBC normally shows low counts for white blood cells, red blood cells, and platelets in HCL patients. However, if large numbers of hairy cells are in the blood stream, then normal or even high lymphocyte counts may be found.

On physical exam, 80–90% of patients have an enlarged spleen, which can be massive. This is less likely among patients who are diagnosed at an early stage. Peripheral lymphadenopathy (enlarged lymph nodes) is uncommon (less than 5% of patients), but abdominal lymphadenopathy is a relatively common finding on computed tomography (CT) scans.

The most important lab finding is the presence of hairy cells in the bloodstream. Hairy cells are abnormal white blood cells with hair-like projections of cytoplasm; they can be seen by examining a blood smear or bone marrow biopsy specimen. The blood film examination is done by staining the blood cells with Wright's stain and looking at them under a microscope. Hairy cells are visible in this test in about 85% of cases.

Most patients require a bone marrow biopsy for final diagnosis. The bone marrow biopsy is used both to confirm the presence of HCL and also the absence of any additional diseases, such as Splenic marginal zone lymphoma or B-cell prolymphocytic leukemia. The diagnosis can be confirmed by viewing the cells with a special stain known as TRAP (tartrate resistant acid phosphatase). More recently, DB44 testing assures more accurate results.

It is also possible to definitively diagnose hairy cell leukemia through flow cytometry on blood or bone marrow. The hairy cells are larger than normal and positive for CD19, CD20, CD22, CD11c, CD25, CD103, and FMC7. (CD103, CD22, and CD11c are strongly expressed.)

Hairy cell leukemia-variant (HCL-V), which shares some characteristics with B cell prolymphocytic leukemia (B-PLL), does not show CD25 (also called the Interleukin-2 receptor, alpha). As this is relatively new and expensive technology, its adoption by physicians is not uniform, despite the advantages of comfort, simplicity, and safety for the patient when compared to a bone marrow biopsy. The presence of additional lymphoproliferative diseases is easily checked during a flow cytometry test, where they characteristically show different results.

The differential diagnoses include: several kinds of anemia, including myelophthisis and aplastic anemia, and most kinds of blood neoplasms, including hypoplastic myelodysplastic syndrome, atypical chronic lymphocytic leukemia, B-cell prolymphocytic leukemia, or idiopathic myelofibrosis.

Because the cause is unknown, no effective preventive measures can be taken.

Because the disease is rare, routine screening is not cost-effective.

T-PLL is an extremely rare aggressive disease, and patients are not expected to live normal lifespans. Before the recent introduction of better treatments, such as alemtuzumab, the median survival time was 7.5 months after diagnosis. More recently, some patients have survived five years and more, although the median survival is still low.

T-PLL has the immunophenotype of a mature (post-thymic) T-lymphocyte, and the neoplastic cells are typically positive for pan-T antigens CD2, CD3, and CD7 and negative for TdT and CD1a. The immunophenotype CD4+/CD8- is present in 60% of cases, the CD4+/CD8+ immunophenotype is present in 25%, and the CD4-/CD8+ immunophenotype is present in 15% of cases.

A case has been described as CD20+, CD22+, and CD5-.

It can also be CD5+.

Another case was described as CD45+, CD19+, CD20+, CD5+, HLA-DR+, CD10-, CD23+/-, CD38+ and FMC7-.

B-cell prolymphocytic leukemia is a more aggressive, but still treatable, form of leukemia. The malignant B cells are larger than average. The name is commonly abbreviated B-PLL.

It can involve deletions from chromosome 11 and chromosome 13.

It has been suggested that some cases may represent a variant of mantle cell lymphoma.

It has a relatively poor prognosis.