In light of previous reports of alterations in 5′-nucleotidase activity in neoplastic conditions, 5′-nucleotidase activity was examined histochemically in tissue sections and quantified biochemically in extracts of human hyperplastic prostates and prostatic carcinomas obtained surgically. The 5′-nucleotidase activities per mg protein in extracts of 29 prostatic carcinomas were lower (P < 0.0005) than in extracts from 10 samples of benign prostatic hyperplasia. The 5′-nucleotidase activity per mg protein in extracts of prostatic carcinoma from the 29 patients correlated (R = -0.369, P = 0.049) with the degree of histological differentiation; the extracts of poorly differentiated carcinomas contained low levels of 5′-nucleotidase. When age and histological differentiation (Gleason's grade) were adjusted, the enzyme activity per mg protein correlated (R = 0.242, P = 0.004) with patient survival. When all three parameters were considered together, i.e., histological grade of the tumor, 5′-nucleotidase extracted from the tumor, and age of the patient, they were found to be mutually complementary for the prediction of patient survival (R = 0.388, P = 0.0001). To our knowledge, this is the first report that prostatic epithelium expresses 5′-nucleotidase; further work will be required to define the reasons for the high levels of activity observed in prostates without cancer and for the decrease in the activity in prostatic carcinoma.