A multicentre clinical trial was carried out in order to evaluate the effect of interferon (IFN) in patients with multiple myeloma. Patients (n = 120) who had shown response to conventional intermittent melphalan-prednisone induction therapy, and achieved a plateau phase, were randomized at that point to receive either interferon alfa-2b in a dose of 5 million units (MU) three times per week or no therapy. This report presents the results of an interim analysis, performed when the patients had been followed for a median of 20 months. The duration of the plateau phase was significantly longer in the IFN arm (59 weeks), compared to the no therapy arm (26 weeks). A total of 34 deaths have occurred, 13 in the IFN arm and 21 in the no therapy arm. In spite of the high median age of the patients studied (70 years), most patients were able to tolerate a full or only slightly reduced IFN dose.