In May 1972, the Cancer and Leukemia Group B initiated a randomized study comparing the effectiveness of CCNU and methyl-CCNU in patients with advanced malignant lymphomas, including Hodgkin's disease (HD), lymphosarcoma (LYS) and reticulum cell sarcoma (RCS). A single dose of 100 mg/m2 of CCNU or 150 mg/m2 of methyl-CCNU was given orally every 6 weeks. In patients with leukopenia or thrombocytopenia, due to prior treatment, this dose was reduced to 70 mg/m2 of CCNU and 100 gm/m2 of methyl-CCNU. Of 109 evaluable patients, 60 received CCNU and 49 received methyl-CCNU. Response rates (complete and partial) to CCNU and methyl-CCNU were respectively 42% (14/33) and 15% (3/20) in HD, 21% (3/14) and 21% (3/14) in LYS, 15% (2/13) and 27% (4/15) in RCS. Responses to methyl-CCNU, but not to CCNU, were seen only in patients who developed significant hematologic toxicity. Responses to both drugs were generally of short duration due to the advanced stage of the disease. Renal, hepatic or neurological toxicity was not observed. In conclusion, CCNU proved to be superior to methyl-CCNU for the treatment of advanced HD. CCNU was also observed to be of higher activity in Hodgkin's than in non-Hodgkin's lymphomas.