We describe a 72-year-old woman who was diagnosed with asymptomatic multiple myeloma (MM) while being treated for Philadelphia (Ph)-positive chronic myeloid leukemia (CML) with imatinib mesylate (400 mg/day). The diagnosis of CML was based on the presence of the Ph chromosome and chimeric BCR-ABL messenger RNA. Three months after starting imatinib mesylate treatment, the patient achieved a complete cytogenetic response. However, bone marrow analysis at that time demonstrated plasmacytosis, and paraprotein (IgG, kappa-type) was also detected. Hypercalcemia, renal failure, anemia, and bone lesions were not observed, which suggested that asymptomatic MM had developed. The coexistence of CML and MM is an extremely uncommon event that has only been reported in 12 cases. We discuss the relationship between CML and MM.