Assessing the QOL in cancer clinical trials is becoming increasingly important. However, a suitable device of assessment of QOL has not been developed yet for prostate cancer patients in Japan. We tried to assess the QOL of prostate cancer patients using the EORTC questionnaire translated in Japanese, and examined its validity and reliability. Thirty-six patients filled in a questionnaire. The reliability of this device was confirmed by the results of test-retest reproducibility. Good correlation was shown between the results and patients performance status, and between the results and clinical stages, which support the validity of the device. As for the results of assessment of QOL, these patients were severely damaged in sexuality. Those factors of functional status, physical symptoms and fatigue/malaise were closely related to disease activity and clinical stage.