The relationship between the serum values of prostate specific antigen (PSA) and the extent of histological prostatitis was investigated in 42 patients undergoing transurethral resection of the prostate for benign hyperplasia (BPH) without clinical evidence of prostatitis. Histological prostatitis was divided into three groups: acute, chronic-active, and chronic-inactive inflammation. The extent of histological prostatitis was expressed as the number of prostatic acinar and ductal glands with inflammatory infiltrate per total number of glands (%). The serum PSA values significantly correlated with the extent of acute and chronic-active prostatitis (correlation coefficient r = 0.765 and 0.656, P < 0.01). A relationship between PSA values and the extent of chronic-inactive prostatitis was not found. In the immunohistochemical study, prostatic epithelial cells with acute and chronic-active inflammation showed negative staining for PSA antigen. These results indicate that histological acute and chronic-active prostatitis is considered an important factor for inducing the high increase in serum PSA values via the leak phenomenon.