The established treatment for multiple myeloma (MM) comprises induction with infusional chemotherapy, high dose chemotherapy (HDC) and autologous transplantation followed by maintenance interferon. On relapse, patients (pts) are reconsidered for this however some are unsuitable and in this situation the therapeutic options are limited. Between June 1995 and May 1997, 14 pts with previously treated relapsed or refractory MM were recruited. Using a prospective database, the tolerability and efficacy of chronic low dose oral idarubicin was evaluated. The median age of pts was 63 years. All had received previous anthracycline in the form of infusional cVAMP chemotherapy. 11/14 had received previous HDC. Median time from diagnosis to commencing idarubicin was 77 months. 10 mg idarubicin was administered 3 times/week for 3 weeks of a 5 week cycle. The maximum number of courses was 6. Three pts completed 6 courses, 5 pts 3 courses, 2 pts 2 courses and 4 pts 1 course. The reasons for stopping treatment were death due to progressive disease (PD) in 7 pts, persistent thrombocytopenia in 2 pts, PD in 1 pt and 1 pt suffered a cerebral infarction not considered to be related to the idarubicin therapy. Two pts showed evidence of response, neither amounting to a partial response. One had stabilisation of paraprotein with a reduction in bone marrow infiltration (47% to 7% plasma cells), the other had a reduction in bone marrow infiltration after 3 course but an increase after 6 courses. In total forty-one courses of treatment were administered. Grade 3/4 haematological toxicities were noted in a minor fraction of cases and were as follows: anaemia 6/41, neutropenia 10/41 and thrombocytopenia 11/41. Our data therefore shows a minor response in 2/14 (14%) of heavily pretreated patients with MM, without evidence of severe toxicity. It provides the rationale for using oral idarubicin as either single agent or in combination therapy for patients earlier on in their disease course especially if they are unsuitable for standard therapy.