The case of a 72-year-old woman with chronic myelogenous leukemia in blast phase (BP) with hypercalcemia is reported. Bone x-ray examination revealed multiple osteolytic lesions throughout the body. The serum level of parathyroid hormone-related protein (PTHrP) was elevated, and PTHrP messenger RNA (mRNA) was detectable in the peripheral blood mononuclear cells (PBMNC) at BP but was not detectable at chronic phase (CP).Treatment with conventional chemotherapy did not completely control either serum calcium level or serum PTHrP level. Treatment with imatinib mesylate (imatinib) alone rapidly normalized these parameters in parallel with a decrease in the number of blast cells. The treatment also maintained the patient in good condition for approximately 3 months, even though the number of blast cells, serum calcium level, serum PTHrP level, and PTHrP mRNA level increased at the terminal stage. Mutations of the p53, K-Ras, and BCR-ABL genes in PBMNC at BP were absent. A noteworthy feature in this patient was that PBMNC at BP but not at CP showed high Lyn mRNA expression. Taken together the findings showed that production of PTHrP by blast cells was favorably controlled by imatinib therapy alone. Imatinib may prolong survival time at BP even though the patients have the complication of PTHrP-mediated hypercalcemia.