We studied the clinical and pathological features of six cases of non-Hodgkin's lymphoma (diffuse undifferentiated in four cases and diffuse histiocytic in two cases) occuring in patients treated for Hodgkin's disease. All six patients had received both radiation and chemotherapy. Abdominal or gastrointestinal involvement was present in five of the six cases. None of the patients had evidence of Hodgkin's disease when the diagnosis of non-Hodgkin's lymphoma was made. Five of the six patients were among a study group of 579 patients with Hodgkin's disease, prospectively followed since diagnosis. At 10 years the actuarial risk of development of non-Hodgkin's lymphoma in this study group is 4.4 per cent (1.2 to 15.0) (per cent probability with 95 per cent confidence limits) and is similar to that of developing acute leukemia: 2.0 per cent (0.3 to 12.9). Non-Hodgkin's lymphoma is a second tumor that may occur late in the course of patients treated for Hodgkin's disease--particularly in patients who have received both radiation therapy and chemotherapy. Like acute leukemia, non-Hodgkin's lymphoma may be another cancer that represents a substantial late risk of combined-modality therapy.