Polyamine metabolism in prostatic tissue, which may give rise to prostatic hyperplasia by inducing cell proliferation, is controlled primarily by two enzymes: arginase and diamine oxidase (DAO). Arginase catalyzes the synthesis of ornithine, the precursor for polyamines from arginine, whereas DAO catalyzes the oxidation of diamines. such as spermine and spermidine, to a much less active compound called putrescine. In this study, we evaluated arginase and DAO activities in prostate tissues of 50 patients with benign prostatic hyperplasia and in 23 patients with prostatic carcinoma. Both DAO and arginase activities were found to be elevated in cancer tissues as compared to benign prostatic hyperplasia (fivefold and twofold, respectively; P < .001). A strong inverse correlation between arginase and DAO activities was observed in those in the cancer group (r = -0.65; P < .001). Gleason grades of prostatic carcinomas were well correlated with both arginase and DAO activities. Results suggest that DAO and arginase might play an essential role in the mechanism of prostatic disease process by modulating polyamine levels.