Over the course of a 2-year period (September 1987 to September 1989) a total of 432 patients with multiple myeloma were registered in a study comprising 39 Swedish and one Italian clinics. Six of these were university hospitals, the others were county (community) hospitals. A total of 308 patients fulfilled the eligibility criteria of the protocol, and were started on traditional intermittent melphalan-prednisone therapy. Response, defined as a reduction in the concentration of the M-component in serum or urine, was observed in 58% of patients, and a clearly defined plateau phase was noted in 38% of the cases. Of the patients who achieved a plateau phase, 120 individuals (with a median age of 71 years) were at this point randomized between no further therapy and continuous interferon-alfa-2b at a dose of 5 x 10(6) IU subcutaneously three times per week. At relapse the interferon therapy was discontinued, and melphalan-prednisone restarted. Only preliminary data and interim results are at this point available. The final evaluation of the study will be performed after June 1, 1991. The treatment arms are comparable with regard to age, stage according to Durie-Salmon, renal function, immunoglobulin class, and type of response to the cytostatic therapy (rapid versus slow). A median number of six courses of melphalan-prednisone were administered before randomization. For those patients in whom pretreatment serum beta-2-microglobulin was analyzed, no difference in the median values between the treatment arms was found. The median observation time from randomization is 20 months. At the time of this analysis, 85 had relapsed: 33 of 59 (56%) in the interferon arm, and 52 of 61 (85%) in the no therapy arm. An interim analysis of the duration of the plateau phase (from the time of randomization) was performed in October 1990, showing a highly significant difference between the two treatment arms (P less than .001). The median duration of plateau was 59 weeks in the interferon arm, and 26 weeks in the no therapy arm. To date there have been 34 deaths, 13 in the interferon arm and 21 in the no therapy arm. All patients in the no therapy arm died from multiple myeloma (n = 19), or from another cause, but with the myeloma in an uncontrolled, progressive state (n = 2). In the interferon arm, only 10 patients died from progressive myeloma, and four of these either did not start interferon therapy at all (n = 1), or discontinued therapy early (n = 3).(ABSTRACT TRUNCATED AT 400 WORDS)