In a Nordic multi-centre trial, 583 previously untreated multiple myeloma patients were randomized to receive melphalan-prednisone or melphalan-prednisone+ interferon alpha-2b at a dose of 5 million units subcutaneously, 3 d/week. A quality-of-life study was integrated into the trial, using the EORTC QLQ C-30 questionnaire supplemented with 11 questions concerning interferon toxicity. The questionnaire was completed prior to treatment and after 1, 6, 12, 24, 36 and 48 months. 90% of the patients participated in the quality-of-life study, and 83% completed all questionnaires submitted to them. During the first year of treatment the patients on interferon reported significantly more fever, chills, dry skin, fatigue, pain, nausea/vomiting and appetite loss than the control patients. There was a moderate reduction of the global quality-of-life score and slight, non-significant, reductions of physical, emotional, cognitive, social and role functioning scores. After the first year there were no statistically significant differences in any toxicity, symptom or quality-of-life score, except for an increased frequency of dizziness in the interferon group. As only 60% of the patients remained on interferon after 24 months, our data probably underestimate the potential toxicity of the drug. Although there was no significant survival benefit for the interferon patients, a 5-6 months prolongation of the response and plateau phase duration was observed. However, by intention-to-treat analysis, there was no late quality-of-life benefit for the interferon patients to compensate for the early impairment. Thus, the clinical significance of the plateau-phase prolongation is uncertain.