Many agents used in cancer chemotherapy are known carcinogens. However, few secondary malignancies have been definitely linked to chemotherapy, since studies on this problem are complicated by methodological problems. A causal relationship has been established between alkylating agents and leukaemia and between cyclophosphamide and bladder cancer. The risk of leukaemia peaks at 5-10 years after beginning of chemotherapy and declines steadily after its end. The interaction between chemotherapy and radiotherapy has not been fully clarified, nor has the leukaemogenic potency of individual drugs, although combinations without nitrogen mustard seem to entail a lower risk. Other tumours reported at increased incidence, in particular among Hodgkin's disease patients, for whom a carcinogenic effect of chemotherapy seems plausible, are non-Hodgkin's lymphoma and lung cancer. Other secondary solid tumors have also been reported, but for none of them an independent effect of chemotherapy has been demonstrated.