Of 602 patients treated for non-Hodgkin's lymphomas, 9 developed overt acute nonlymphocytic leukemia or preleukemia with refractory cytopenia and cytogenetic abnormalities of the bone marrow. A Kaplan-Meier estimate of the cumulative probability of leukemia complications was 6.3 ± 2.6% (mean ± SE) 7 years after start of treatment. All 9 patients with leukemic complications belong to a major subgroup of 498 patients treated with alkylating agents, predominantly cyclophosphamide. The risk of leukemic complications in this subgroup was compared with the risk in 312 patients treated with other alkylating agents for Hodgkin's disease and with the risk in 553 patients treated with dihydroxybusulfan for ovarian carcinoma. Cumulative 9-year risks were 8.0 ± 3.3%, 12.8 ± 3.5%, and 7.1 ± 1.9%, respectively. The general risk of secondary leukemia after long-term treatment with alkylating agents ranges from 1% to 1.5% per year from 2 to at least 9 years after start of treatment.