Between January, 1982 and May, 1988, transurethral resection of the prostatic gland diagnosed as clinically benign prostatic hypertrophy was performed on 461 patients at Yokohama Minami Kyosai Hospital. Thirty-eight cases of incidental carcinoma (8.2%) were found by histopathological examination. The patients with well and moderately differentiated adenocarcinoma decreased in proportion to the increase in the number of the tumor chips. Of the 10 patients with more than 11 tumor chips, 7 patients (70%) had histologically poorly differentiated adenocarcinoma. The cases of INF alpha were only found in the group with the ratio of tumor chips to examined chips of less than 5%. The ratio of the cases of INF gamma increased with the increase in the ratio of tumor chips. Uniform substaging criteria of incidental prostatic carcinoma (stage A) obtained from the tissue by transurethral resection should be established.