High-dose chemotherapy (melphalan) with autologous marrow stem cell support (AMSCS) results in high response rates in multiple myeloma (MM), with up to 50% of patients achieving complete remission. However, these remissions are generally not durable. As the cytokine interferon alpha has been shown to prolong partial response following conventional chemotherapy, this trial was conducted to evaluate its role following high-dose chemotherapy. 85 patients were randomly assigned to maintenance treatment with interferon alpha, 3 x 10(6) units/m2 subcutaneously three times weekly until relapse or no further treatment following recovery from high-dose chemotherapy (melphalan 140-200 mg/m2 or busulphan 16 mg/kg) combined with AMSCS. At 5.8 years following the accrual of the last patient in this trial, 38 patients had died, 17 in the interferon arm and 21 in the control arm. The median progression-free survival (PFS) in the 42 patients randomized to interferon alpha was 46 months versus 27 months in the controls. Both overall survival and PFS, which were highly significant at median follow-up of 52 months, have now ceased to be significant, because most patients have ultimately succumbed to their disease. Interferon was tolerated by the majority of patients with very good compliance. Toxicity consisted mainly of flu-like symptoms and malaise which were usually self-limiting. The results of such a pilot study should be carefully interpreted and the benefits of interferon should be confirmed in larger multicentre studies in the setting of minimal residual disease following autologous transplantation.