We reviewed 251 consecutive adult patients with Hodgkin's disease treated at the Division of Hematology, Policlinico S. Matteo, Pavia, from January 1970 to December 1979, to assess the risk of development of acute leukemia. The median time of follow-up was 48 months (range 6-135). No leukemia occurred in 88 patients treated with radiotherapy or chemotherapy alone. Six acute non-lymphoid leukemias occurred in the group of 163 patients treated with MOPP and radiotherapy (crude rate of leukemia of 7.5 per 1000 person-years at risk). All cases were in clinical remission and off therapy; the latent period from initiation of therapy to onset of leukemia ranged between 30 and 90 months. The actuarial probability of leukemia at five and seven years was 2.9 and 4.7% for the entire group of patients, and 3.8 and 5.8% for the combination therapy group. All leukemias , except one, had a preleukemic phase lasting 1-12 months, with cytopenia and dysplastic marrow. The median survival after leukemia was 4.7 months.