Hairy cell leukemia (HCL) is a rare small lymphoid (B-cell) neoplasm that is characterized by accumulation of neoplastic cells in the spleen (splenomegaly) and cytopenias (monocytopenia is common). Lymphadenopathy is not considered a typical feature of HCL. The median age of diagnosis is 55-60 years with a strong male predominance; male to female ratio is 4:1 and it is more common among White individuals. HCL does not occur in children. Rare cases of familial HCL have been reported. 

Signs and symptoms are usually related to underlying cytopenias (anemia, thrombocytopenia, monocytopenia, neutropenia) and splenomegaly (80%-85% of patients), as well as constitutional symptoms (eg, fatigue, weight loss, night sweats).

Peripheral blood will show cytopenias and/or circulating neoplastic cells, or hairy cells. They are small to intermediate sized lymphoid cells with oval nucleus and abundant pale blue-gray cytoplasm with indistinct outline with varying numbers of projections, imparting a "hairy" appearance.

Bone marrow biopsy will show lymphoid infiltrate made up of hairy cells. Their abundant cytoplasm makes the nuclei widely separated, giving a "fried egg" appearance. Fibrosis is common, and bone marrow aspirates are often "dry taps" because of this.

HCL is an indolent disease. Patients frequently experience relapse. While treatment is not curative, survival with modern therapy is only slightly lower than the general population.