This cooperative study was sponsored by the National Prostatic Cancer Project to determine the usefulness of serum acid phosphatase levels as a predictive indicator with regard to performance status, sites of metastases, response to treatment, and survival in patients with advanced prostatic carcinoma. The results indicate that survival was significantly shorter for those patients who had elevation of thier on-study (pretreatment) total serum acid phosphatase ler cent reduction of primary tumor mass, relief of pain, and acid phosphatase activity. No correlation could be demonstrated between serum acid phosphatase and performance status, site of metastases, and other criteria of response to therapy. It is concluded that this test as currently determined spectrophotometrically at this stage of disease and if employed alone is not sufficient to allow for total evaluation of the response of therapy. It is, however, helpful when used in correlation with the previously mentioned positive factors.