• Abnormal levels of serum acid phosphatase specifically of prostatic origin can provide the key to accurate diagnosis of cancer of the prostate. The presence of normal levels of "total" serum acid phosphatase cannot be considered sufficient evidence to rule out prostatic cancer, especially where there are other clinical signs of this disease. Where there is a questionable diagnosis of prostatic cancer, the administration of testosterone 50 mg. three times a week, may be followed by a significant rise in "prostatic" serum acid phosphatase level, by stimulation of the tumor growth. Five unsuspected cases of carcinoma of the prostate were diagnosed by abnormal prostatic serum acid phosphatase values among 233 males admitted to the hospital during a two-year period. A normal prostatic serum acid phosphatase level is equally important from a diagnostic standpoint.