From 1977 to 1982, 377 patients with small cell lung cancer were treated at Vanderbilt University Medical Center. All patients received combination chemotherapy consisting of cyclophosphamide, doxorubicin, and vincristine (CAV) with or without methotrexate, etoposide, and/or hexamethylmelamine. Thoracic and/or prophylactic cranial irradiation was administered to 159 (42 percent) and 192 (51 percent) patients, respectively. Acute nonlymphocytic leukemia was observed in two patients at 22 and 81 months from the start of therapy. The relative risk of leukemia was 154 (95 percent confidence limit, 38 to 293). A Kaplan-Meier estimate of the cumulative probability of leukemia was 1.9 +/- 1.4 percent seven years after the start of treatment. The relative risk of leukemia is significantly increased in this group of patients (p less than 0.0001). Acute nonlymphocytic leukemia is a long-term complication of small cell lung cancer therapy.