Six of 796 patients treated with intensive combination chemotherapy for small cell carcinoma of the lung developed overt acute nonlymphocytic leukemia (ANLL) (three patients) or preleukemia with severe refractory cytopenia and clonal cytogenetic abnormalities in bone marrow cells (three patients). The latent period to development of preleukemia or leukemia was less than two years in four of the six patients. The cumulative risk of preleukemia and leukemia according to a Kaplan-Meier estimate was 14.0% +/- 6.9% (mean +/- SE) four years after the start of treatment. The relative risk of overt ANLL was 77, since three cases were observed v 0.039 cases expected, based on the age- and sex- specific incidence of acute nonlymphocytic leukemia in the general Danish population. The risk of secondary solid tumors was not increased. The possible causes of the exceptionally early appearance and very high cumulative risk of leukemic complications found in the present study, as compared to previous experience in other malignant diseases, is discussed, including the implications for future therapy of patients with small cell lung cancer.