Thirteen hairy-cell leukaemia patients were treated with IFN-beta (6 X 10(6) IU/m2) for 7 days, alternate weeks, for three cycles. IFN-beta was then continued at the same dose twice a week for 24 weeks. Treatment was discontinued in 2 non-responders and 2 partial responders (1 haem PR, 1 path PR) because of complications unrelated to IFN. The objective response in the nine patients who completed therapy was 66% (1 CR, 3 path PR and 2 haem PR); 2 patients achieved MR. Responses lasted from 5 to 45+ months. Four newly diagnosed patients and 3 in relapse after discontinuation of IFN-beta therapy (6 X 10(6) IU/m2), were treated with a lower dose of IFN-beta (2 X 10(6) IU/m2). The objective response to this dose was 57% (3 path PR, 1 haem PR). Another patient obtained MR. No patient has relapsed 6-12 months after therapy discontinuation. IFN-beta was well tolerated, especially at the lower dose and no chronic toxicity was observed. Therefore IFN-beta may be suggested as an alternative treatment for HCL.