We report a 52-year-old man with chronic myelogenous leukaemia (CML) who developed hypercalcaemia in blast crisis. There was a high serum level of parathyroid hormone-related protein (PTHrP). The leukaemia cells secreted PTHrP into culture medium when cultured in vitro and were shown to express PTHrP mRNA by reverse transcription-polymerase chain reaction. Transplantation of the leukaemia cells into nude mice resulted in the production of Ph1 chromosome-positive tumours which caused increased levels of calcium and PTHrP in the blood. These results indicated that the hypercalcaemic event in the patient was induced by PTHrP of CML cell origin.