The descriptive epidemiology of hairy cell leukaemia is reported from a specialized register of haematological malignancies covering approximately one-third of England and Wales. The overall incidence of hairy cell leukaemia at 2.9 per million persons per year is similar to that recorded in America. There is a marked male preponderance (4.0 compared with 1.7 per million per year for females). A case-control study on hairy cell leukaemia was conducted in the Yorkshire and Trent Regional Health Authority areas. 50 cases and 95 controls were identified, and interviewed using a structured questionnaire. Previous results on the aetiology of hairy cell leukaemia were not supported by this study, with the exception of an association between hairy cell leukaemia and exposure to organic solvents, petrochemicals and related products.