The use of interferon for the induction treatment of multiple myeloma has been shown to be effective in about 20 percent of patients. We studied its effects on long-term survival when it was used for maintenance treatment. Between April 1985 and May 1988, 101 patients with symptomatic multiple myeloma who had had a substantial objective response or a lesser objective response with disappearance of symptoms ("disease stabilization") after 12 courses of induction chemotherapy were randomly assigned to receive recombinant interferon alfa-2b as maintenance therapy (n = 50) or to receive no treatment (n = 51). As of December 1989, 66 of the 101 patients have relapsed (25 given interferon and 41 not treated). The median duration of response (from the time of randomization) was 26 months in the patients given interferon and 14 months in the untreated patients (P = 0.0002). A total of 37 patients have died (14 given interferon and 23 not treated). The median duration of survival (from randomization) was 52 months in the interferon group and 39 months in the control group (P = 0.0526). Among the patients who had had a substantial objective response to induction chemotherapy, the difference in survival time was statistically significant (P = 0.03526). Interferon had to be stopped because of toxic effects in 3 of 12 patients initially treated with 10 MU (megaunits) per square meter of body-surface area. After the dose was reduced to 3 MU per square meter, the only toxic effect was a mild influenza-like syndrome lasting two to three weeks. We conclude that maintenance treatment with interferon prolongs response and survival in patients with multiple myeloma who have responded to conventional induction chemotherapy.