Since July 1984, eight patients with advanced hairy cell leukemia have received treatment with recombinant alpha A interferon. At commencement of interferon, seven patients had progressive cytopenia, and one was in leukemic phase (greater than 20 x 10(9)/L circulating hairy cells). All patients had had previous splenectomy. Interferon was administered subcutaneously. The initial dose was 3 x 10(6) U/day, continued until peripheral counts stabilised. Subsequently, patients received 6 x 10(6) U/day, 9 x 10(6) U/day, and finally 12 x 10(6) U/day. The dose increases proceeded every 8-12 weeks, as tolerated. Seven patients had an objective response. There were four complete remissions, two partial remissions, and one minor response. Complete remission was documented only in patients on at least 6 x 10(6) U/day for 12 weeks. The median time to complete remission was 40 weeks (range 35-53). Normalisation of peripheral blood counts preceded histologic marrow improvement. The median times for response (platelets greater than or equal to 100 x 10(9)/L, hemaglobin greater than or equal to 12 gm/dL, neutrophils greater than or equal to 1.5 x 10(9)/L), were six to eight and 17 weeks, respectively. Toxicity included myelosuppression during the first four weeks of therapy. With increasing doses of interferon, myelosuppression did not recur. A transient, mild, flu-like syndrome affected all patients. Two patients developed asymptomatic transaminitis at doses greater than 6 X 10(6) U/day. This resolved with dose reduction. In one case impotence was reported during the first four weeks of each interferon level.(ABSTRACT TRUNCATED AT 250 WORDS)