Hairy-cell leukaemia (HCL) is an established clinicopathological entity typified by a chronic picture of substantial splenomegaly, less frequent hepatomegaly, and only inconspicuous lymphadenopathy. Central to the diagnosis is the pathognomonic hairy cell (HC), a dis- tinctive mononuclear cell that circulates in varying numbers and infiltrates a variety of organs, including the bone marrow and spleen. Peripheral pancytopenia is often present, and circulating HCs may be infrequent. HCs are invariably present in the bone marrow; but, per- haps because of the extensive fibrosis characteristic of the disease, the marrow is frequently difficult to aspirate, and confirmation of the diagnosis may require histopathological ex- amination of biopsy material. The incidence of the disease has been given as 2% ofleukaemias [29, 319] or as approximately 1 % of identifiable lymphomas [230]. Our own experience of collecting cases in the United Kingdom leads us to expect that at any given time one patient with HCL will be attending a clinic serving some 150,000 people.