We attempted treatment with methyl 6-[3-(2-chloroethyl)-3-nitrosoureido]-6-deoxy-alpha-D-glucopyranoside (MCNU), a novel nitrosourea derivative, in a 55-year-old man with advanced-stage primary myelofibrosis. MCNU was given intravenously at a dose of 50 mg once a month. Following MCNU treatment, his anemia and splenomegaly improved markedly. An increased dose of MCNU (100 mg, once a month) was even more effective for relieving the symptoms. Severe side effects resulting from this therapy, such as leukocytopenia or thrombocytopenia, were never observed. These observations indicate that MCNU treatment may be a beneficial management of advanced-stage primary myelofibrosis.