A retrospective analysis was performed to determine the incidence and clinical features of acute myelogenous leukemia/myelodysplastic syndrome (AML/MDS) developing in chronic lymphocytic leukemia (CLL) patients. AML/MDS occurred in 3/1374 CLL patients seen at a single institution between 1972 and 1992. The median follow-up exceeded 7 years and 72% of these patients had received prior alkylating agent therapy. The expected number of AML/MDS developing in a general population of the same size was 1.2 as calculated from the National Cancer Institute's Surveillance, Epidemiology, and End Results (SEER) Program data (observed-to-expected ratio = 2.49; 95% confidence interval = 0.9-7.3; p = 0.12). Although, not included in the incidence calculation, four patients with CLL were referred at the time of development of AML/MDS. CLL and AML/MDS were diagnosed concurrently in two patients. Three of the patients had received no prior alkylating agents. The median survival was 17 months in patients who had received no prior treatment, and 5 months in those who had received prior chemotherapy. Our results suggest that patients with CLL in whom AML/MDS develops have similar prognoses to other patients with AML/MDS. Furthermore, this analysis does not provide evidence for a heightened risk of AML/MDS in CLL patients, despite treatment with known leukemogenic agents.