After half a century of clinical application, the usefulness of prostatic acid phosphatase (PAP) as a marker for carcinoma of the prostate is being challenged. Historically the first of the "serum markers" for the presence of a human cancer,1 acid phosphatase is still measured in clinical laboratories all over the world. The method has changed to incorporate immunoassay techniques — even monoclonal antibodies2 — but the clinical use has not changed. PAP is measured to detect the presence of metastatic disease and to monitor its response to systemic therapy, usually androgen deprivation. Attempts to use PAP assays of increased sensitivity . . .