A new water-soluble nitrosourea derivative, methyl 6-[3-(2-chloroethyl)-3-nitrosoureido]-6-deoxy-alpha-D-glucopyranoside (MCNU), was found to be useful for the treatment of chronic myelogenous leukemia (CML) in the chronic phase. To compare the efficacy of MCNU with that of busulfan, patients were randomized. In the 40 patients administered MCNU, the median time to the achievement of a complete remission (CR) was 50 days. This value was shorter than that observed in 37 patients administered busulfan (126 days, p less than 0.05). There were no differences in the rate of CR achieved, mortality, median time to the onset of blast crisis (BC), BC rate, or survival rate during the observation period. The overall incidence of side effects was higher for MCNU (31%) than for busulfan (15%), but the symptoms were mild, transient and tolerable for most patients. These results suggest that MCNU is a safe and valuable addition to the therapeutic repertoire for the control of CML.