The occurrence of acute leukaemia and other secondary neoplasms in 432 patients treated with conventional chemotherapy for multiple myeloma was analysed after a follow-up period of 11-19 yr (mean 16 yr). The number and organ-specific distribution of observed solid neoplasms was close to that expected in the general population. Non-Hodgkin's lymphoma developed in three patients (expected 0.7, p = 0.19). Acute leukaemia was diagnosed in 14 patients with an actuarial risk of 9.8% at 9 yr. No further cases were diagnosed thereafter. The average numbers of courses (26.2 vs. 25.5) and cumulative doses of melphalan (1440 and 1400 mg) were similar in patients with or without acute leukaemia. It seems possible that the advanced stage of multiple myeloma is more vulnerable to the leukaemogenic effect of melphalan compared with the earlier stages.