Hairy Cell Leukemia is an uncommon form of chronic leukemia that, like CLL, affects B-lymphocytes. The median age at presentation of HCL is 55 years, and there is a 5 to 1 male predominance.

Most patients with HCL present with gradual onset of fatigue, others experience symptoms related to spleen enlargement, and yet others come to attention because of infections. Upon physical examination, an enlarged spleen (splenomegaly) is almost invariably present and may be massive. The liver is enlarged in approximately fifty percent of cases, but swollen lymph nodes are uncommon.

Laboratory findings include B-lymphocytes, which have characteristic "hair-like" projections when examined under the microscope and reduced numbers of all types of blood cells (pancytopenia). On immunophenotyping, the leukemic cells co-express the antigens Cd11c and CD22.

HCL is very responsive to treatment with cladribine (Leustatin, 2CdA) and pentostatin (Nipent, 2'-deoxycoformycin).