A total of 764 patients with Hodgkin's disease treated with radiotherapy (RT) or chemotherapy or both were reviewed 3-186 months (median 43 months) after initial treatment to assess the incidence of second malignancies. Incidence of solid tumours and acute non-lymphoblastic leukaemia (ANLL) were calculated by a life-table method and percentages of patients affected derived from life-table plots. Within 10 years after initial treatment the overall incidence of second solid tumours was 7.3%, and over a comparable period 2.4% of patients developed ANLL. Solid tumours occurred only in patients given RT with or without adjuvant chemotherapy, and ANLL occurred only after treatment with MOPP (mustine, vincristine, procarbazine, and prednisolone) or modified MOPP regimens. Neither solid tumours nor ANLL occurred in patients given ABVD (adriamycin, bleomycin, vinblastine, and dacarbazine). The highest incidence of leukaemia (5.4%) occurred after treatment with extensive RT plus (5.4%) occurred after treatment with extensive RT plus MOPP; hence the benefits of this approach in Hodgkin's disease must be weighed against its carcinogenic potential.