The aim of this study was to analyse the clinical utility of serum bone alkaline phosphatase (BAP) in addition to prostate-specific antigen (PSA) in the staging of newly diagnosed untreated prostate cancer patients. A prospective study was conducted, analysing serum BAP and PSA concentrations in 295 consecutive newly diagnosed untreated prostate cancer patients (T1-4, N0-1, M0-1b), 93 of whom had bone metastases on bone scan. The relationship of each marker with extent of bone disease, the influence of several clinical variables on both serum marker levels, the efficiency in predicting bone metastasis through receiver operating characteristic curves and, finally, the clinical utility in avoiding unnecessary bone scans were determined. Significant differences were found in the serum levels of both BAP and PSA between patients with and patients without bone metastases. Multiple regression analysis showed the extent of bone disease to be the only variable that influenced both serum levels. However, while serum BAP levels showed a statistical relationship with extent of bone disease, serum PSA levels did not. The best prediction of bone scan findings was obtained with the combination of both markers using a cut-off of 20 ng/ml, with positive and negative predictive values of 46.5% and 100%, respectively. This greater efficiency could permit 32.2% of initial bone scans to be avoided. False-positive and false-negative rates of BAP were 7.5% and 14%, respectively. This study suggests that serum BAP levels could play a complementary role in the diagnosis of bone metastasis in prostate cancer patients. This marker could provide useful clinical information on the degree of skeletal metastasis and constitute an easy way of enhancing the clinical utility of PSA. The addition of this marker to PSA in the initial evaluation could permit staging bone scan to be avoided at a PSA range of 10-20 ng/ml, with significant implications for cost saving.