Hairy cell leukemia is a rare hematologic tumor characterized by an accumulation of abnormal B cells. It is classified as a subtype of chronic lymphocytic leukemia (CLL). Hairy cell leukemia accounts for about 2% of all leukemias, with fewer than 2,000 new cases diagnosed each year in North America and Western Europe combined.

In hairy cell leukemia, the "hairy cells" (malignant B lymphocytes) accumulate in the bone marrow, interfering with the production of normal white blood cells, red blood cells, and platelets. Consequently, patients may develop infections related to low white blood cell count, anemia and fatigue due to a lack of red blood cells, or easy bleeding due to a low platelet count. Leukemic cells may gather in the spleen and cause it to swell; this can have the side effect of making the person feel full even when he or she has not eaten much. Hairy cell leukemia is commonly diagnosed after a routine blood count shows unexpectedly low numbers of one or more kinds of normal blood cells, or after unexplained bruises or recurrent infections in an otherwise apparently healthy patient.