Hairy-cell leukemia is a unique lymphoproliferative disorder that has fascinated clinicians and researchers for nearly 3 decades. It can, in most cases, be diagnosed correctly on the basis of a bone marrow biopsy specimen, and the diagnosis is supported by the demonstration of tartrate-resistant acid phosphatase activity in the leukemic cells. Nevertheless, as outlined, rare "variant" cases may pose diagnostic problems, and other lymphoproliferative disorders may have features that closely mimic those of HCL. In virtually all cases of HCL, the hairy cells are of B-cell lineage, representing a relatively mature B cell that is closely related to plasma cells. Treatment of HCL with splenectomy, IFN-alpha, or 2' deoxycorformycin has led to excellent responses in most cases. Much of the progress in our understanding of the nature of this rare leukemia has been made possible through the cooperation of community physicians with referral institutions, or groups of institutions, that are interested in studying HCL. Despite the remarkable advances in the treatment of this disorder over the last 5 years, and the ready availability of IFN-alpha, much remains to be learned regarding the biology of hairy cells, and further advances in therapy still must be made. The needed information can be obtained only through continued referral of patients to centers with expertise and an interest in HCL.