Sixty-seven patients with hematological malignancies and 4 with cancers were evaluated in this study. Standard administration of MCNU was instituted intravenously using 50-100 mg/m2 every 2 or 4 weeks, whereas some cases were treated with a higher dose therapy. Of 10 patients with chronic myelogenous leukemia, 7 achieved complete remission (CR), and 1 achieved partial remission (PR). A good response was also obtained in 9 of 10 patients with polycythemia vera and in all 4 patients with essential thrombocythemia. MCNU was less effective in malignant lymphoma (ML) and multiple myeloma (MM) than in myeloproliferative disorders. Two of 15 patients with ML and one of 21 patients with MM achieved CR, and two with ML and three MM achieved PR. Three patients with lung cancer and 1 with gastric cancer showed no response to MCNU. Delayed anemia, leukocytopenia and thrombocytopenia were observed in 38.7% of patients, and these were regarded as major side effects of MCNU. Nausea, vomiting, anorexia and elevated transaminase were also found in about 24% of patients, but only transiently. Our study indicates that MCNU is useful for chemotherapy of hematological malignancies, especially of myeloproliferative disorders. Therefore, further studies on combination chemotherapy with MCNU should be developed.