One hundred and twenty-seven patients with locally advanced prostatic cancer were evaluated for the presence and progress of bone metastases before and during hormonal therapy, by serial radionuclide imaging and frequent measurement of plasma acid (tartrate-labile) and alkaline phosphatase. For comparison, serial changes in imaging and phosphatases were classified in each patient into one of six groups. Of 71 patients with negative imaging before treatment, 82% had normal alkaline phosphatase levels and 83% had normal acid phosphatase levels. Of 56 patients with bone metastases at presentation, false negative alkaline and acid phosphatase levels were noted in 18% and 36% respectively, though a few patients eventually developed abnormal levels. Serial plasma biochemistry and particularly alkaline phosphatase showed a response to treatment which was not always obvious on imaging. An assessment of the hepatic component of alkaline phosphatase by reference to plasma gamma glutamyl transpeptidase and isoenzyme electrophoresis was helpful in the evaluation of a false positive result but unnecessary where imaging was positive and phosphatase elevated. It is concluded that serial alkaline phosphatase estimation is essential in the follow-up of patients with prostatic cancer and bone metastases, and probably renders serial imaging studies superfluous once the presence of skeletal metastases has been proven. By comparison, acid phosphatase is a much less effective marker.