Carcinoma of the prostate is the commonest malignancy of the genitourinary tract in the male and is frequently associated with metastatic bone disease. Serial isotope bone scans for screening secondary deposits are not cost-effective. We have evaluated the serum prostate markers prostate-specific antigen (PSA) and prostatic acid phosphatase (PAP) as an alternative to conventional serial bone scanning in 129 patients with newly diagnosed prostate cancer over a period of 3 years. Although serum PSA did not reflect local tumour burden at presentation, it was significantly elevated in those who presented with stage D disease (p < 0.01). 45 patients presented de novo with metastatic bone deposits and a further 18 patients developed metastases during the study period. The sensitivity of PSA in detecting secondary deposits at presentation for levels in excess of 100 mug/l was 93.75%, the positive predictive value 95.7% and the negative predictive value for levels less than 5 mug/l was 90.6%. During the follow-up period the sensitivity was 94.4%, the positive predictive value 100% and the negative predictive value 100%, with a median lead time of 3 months in predicting metastases in the 18 patients with progressive disease. When compared with PAP, PSA was found to be a statistically superior marker of bone metastases both at presentation and follow-up (p < 0.05). We recommend that PAP measurements are no longer necessary and should be replaced by PSA, and that serial serum PSA estimations should determine the need for future isotope bone scans in the patient with established prostate cancer.