We studied the progressive factors for incidental carcinoma of the prostate of 38 patients. Effects of anti-androgen drugs for the development of incidental carcinoma were examined. There were no statistical differences with age, tumor size, histopathological grade, or invasion of BPH capsule. However, the tumor progression rate was high for a tumor size of more than 10 mm or poorly differentiated adenocarcinoma. Therefore, tumor progression was related with tumor size and histopathological grade. Of 15 patients with stage A1 disease, two patients had tumor progression. Patients with stage A1 disease are being followed with no treatment, but radical treatment may be necessary for young men. Patients with stage A2 disease must be treated with radical prostatectomy, radiation therapy or hormone therapy case by case. None of the patients who were treated with anti-androgen drugs had stage A1 disease. Patients not treated had stage A1 and A2 diseases. Anti-androgen drugs may have inhibited the development of stage A1 disease.