Allogeneic transplantation may be curative in a proportion of patients with multiple myeloma (MM), but relapse is a major cause of treatment failure. We sought to improve complete remission (CR) rates by the use of alpha-interferon (alpha-IFN) in patients not in CR when evaluated 4 months post-transplant. We report five of 13 evaluable patients undergoing allogeneic sibling BM or PBSC transplantation for MM between 1990 and 1997 who met the criteria for adjuvant alpha-IFN therapy. A starting dose of 3 MU x 3/week was commenced at median time of day +126 (range day +112-224) post-transplant and was well-tolerated. In contrast to other reports we observed no increased toxicity in terms of GVHD compared to those patients not receiving alpha-IFN therapy and only one patient treated with alpha-IFN has developed chronic GVHD. Durable CRs were achieved in two patients within 8 weeks of starting therapy whilst two other patients required a longer course of alpha-IFN to achieve CR (36 weeks and 30 weeks, respectively). One patient whose paraprotein was rapidly rising at the time of alpha-IFN therapy clinically relapsed despite 6 months of treatment. None of the patients who achieved CR following alpha-IFN therapy have relapsed and we conclude that alpha-IFN is a safe and effective adjuvant treatment for some patients in the achievement of CR following allogeneic transplantation for myeloma.