We have evaluated the relation between alkylating agents and leukemic disorders in 3363 1-year survivors of ovarian cancer who were treated in five randomized clinical trials and at two large medical centers. Overall, 28 patients developed acute nonlymphocytic leukemia (expected, 1.2) and 7 developed preleukemia. A 93-fold increased risk for acute nonlymphocytic leukemia was seen in 1794 women treated with chemotherapy; the incidence of leukemic disorders was 7.7/1000 women per year. Risk was highest 5 to 6 years after the first treatment and appeared to decrease thereafter. The use of radiation therapy did not affect risk. The 10-year cumulative risk (mean +/- SE) of acquiring a leukemic disorder was 8.5% +/- 1.6% after treatment with any alkylating agent, 11.2% +/- 2.6% after treatment with melphalan, and 5.4% +/- 3.2% after cyclophosphamide treatment. A dose-response relationship was apparent in 605 women receiving melphalan and suggested in 333 women receiving cyclophosphamide. Women receiving melphalan were two to three times as likely to develop leukemic disorders than were women receiving cyclophosphamide. These data indicate that choice of chemotherapeutic agent and drug dosage may influence significantly the risk for long-term adverse effects of cancer therapy.