Leukemia following chemotherapy for breast cancer was studied among patients diagnosed during 1973-1985 within the population-based tumor registries in the Surveillance, Epidemiology, and End Results Program. Among 13,734 women given initial chemotherapy, 24 developed acute nonlymphocytic leukemia (ANLL) compared to 2.1 expected based on general population rates (observed/expected = 11.5; 95% confidence interval = 7.4-17.1). Overall, 58 excess ANLL occurred per 100,000 women-years at risk for patients treated with chemotherapy. The cumulative incidence was 0.7% at 10 years. Risk remained high over all periods of observation up to 9 years after treatment. Among 7974 women treated only with surgery during 1973 and 1974, a period before the widespread use of adjuvant chemotherapy for breast cancer, ANLL was not significantly increased (observed = 7, expected = 5.1). A case-control study was then conducted in Connecticut to evaluate in more detail the risk associated with adjuvant chemotherapy in the general population. Among 20 cases (17 incident leukemias and 3 deaths due to preleukemia) and 60 matched controls, alkylating agents were linked to an 11.9-fold risk of ANLL and preleukemia (95% confidence interval = 2.6-55). Chemotherapy regimens including melphalan were related to a higher risk of leukemic conditions than those including cyclophosphamide. These data suggest that women in the general population treated with adjuvant chemotherapy for breast cancer are at an increased risk of leukemia, that the risk remains high among long-term survivors, and that risk differs by type of alkylating agent administered.