In a comparative study, we determined the mean serum concentrations of immunoassayable prostatic acid phosphatase (PAP), tartrate-inhibited phosphatase (TP), total acid phosphatase (AcP), and alkaline phosphatase (AP) in different clinical subgroups of patients with histologically proved prostatic carcinoma (PCA). The subgroups were compared with each other and with a reference group of males apparently free of any prostatic disorder. In addition, clinical sensitivities, specificities, and predictive values were calculated to assess the diagnostic value of the different assays. The main results were: (1) Serum PAP concentration measured by immunologic methods best reflected the tumor mass, the presence or absence of metastases, the histologic grade, and the therapeutic efficiency (response) in the patients. (2) The differences in biochemically determined serum TP concentrations were less clear-cut. (3) The serum concentrations of the nonspecific phosphatases AcP and AP were highly elevated in patients with progressed PCA; AP was the highest in patients with palpable tumors and metastases. (4) The sensitivities of each phosphatase were too low for detection of early PCA stages. In conclusion, immunoassayable PAP appears to be the best parameter to monitor advanced PCA disease, and AP may be a useful auxiliary parameter in metastatic PCA.