We addressed in this study whether quantifying the extent of disease on bone scans can predict the disease death of patients with advanced prostate cancer using computer-assisted image analysis. Pretreatment radionuclide bone scans were reviewed in 56 patients with bone metastases from prostate cancer, and the percentage of the positive area on a bone scan (%PABS) was quantified automatically using a personal computer with the NIH Image program for estimation of the accurate extent of metastatic bone lesions on a bone scan. The significance of the %PABS as well as the other known prognostic factors was evaluated using univariate and multivariate Cox proportional hazards analysis. In univariate regression analysis, the %PABS (P=0.0155), serum alkaline phosphatase (P=0.0272), the tumour grade based on biopsy (P=0.044) and the number of bone lesions on bone scans (P=0.0388) were well associated with disease-specific survival. In multivariate analysis, the %PABS (P=0.0155, relative risk ratio 2.603), but not the other factors, was the independent predictor of the disease death. These results suggest that the %PABS is a novel parameter for predicting the prognosis of patients with advanced prostatic cancer.