Waldenström's macroglobulinemia (WM) is an incurable disorder of B cells. Following occasional reports of response to alpha interferon (IFN) and in view of its effectiveness in hairy cell leukemia, we tested this agent in a relatively large group (n = 88) of patients who had an IgM monoclonal component (MC) greater than 10 g/l. Thirty eight patients had a MC > 30 g/l and were classified as Waldenström's macroglobulinemia (WM), while fifty had either WM in an early stage or an IgM monoclonal gammopathy of undeterminated significance (all of them operationally classified as IgM-MGUS). All patients received IFN 3 MU/day for one month and then 3 times/week. Response to treatment was mainly based on MC reduction in two consecutive determinations (> 50%: major response; 25-50%: minor response). Of 36 evaluable WM patients, 12 had a major and 6 a minor response; of 41 evaluable IgM-MGUS patients, 2 had a major and 6 a minor response. In WM patients with a major response, MC reduction was associated with disappearance of hyperviscosity symptoms, raised Hb level and reduced bone marrow lymphoplasmacytosis. At the dose used, tolerance was excellent in the majority of patients; only 15% withdrew from the study due to side effects. Although single cases and very small series have already been reported, no large study collecting quantitative data on the effects of alpha IFN in WM has been published so far. Our results suggest that IFN treatment is not indicated for patients with a low monoclonal component, while it is of clinical benefit in about 50% of patients with IgM > 30 g/l.