A nested case control study was carried out to investigate the association between treatment of patients with Hodgkin's disease (HD) and the risk of developing acute non-lymphocytic leukemia (ANLL). Seven Cancer Centers of the International Cancer Patient Data Exchange System of the UICC participated. A study cohort was selected consisting of 1,681 nonpretreated patients with HD, diagnosed from 1972 through 1978, and followed up through 1984. The median follow-up time was 66 months. Eighteen cases of leukemia were observed in the cohort. The risk of development of ANLL was significantly greater for male than for female patients. The treatment characteristics associated with an increased risk of developing ANLL were extensive radiotherapy, splenectomy and the chemotherapy combination of vincristine, procarbazine and mechlorethamine.