As therapy of cancer becomes more successful, the number of long-term survivors is increasing. In addition, cancer chemotherapeutic drugs are being used as immunosuppressive agents in the chronic treatment of non-neoplastic diseases. The long-range hazards of therapy are a serious consideration in these two groups of patients. This brief review examines the potential oncogenic as well as co-oncogenic hazards of cancer chemotherapy in man. Many of these drugs are oncogenic and∕or co-oncogenic in experimental animals.