To evaluate the usefulness of external beam radiotherapy for patients with prostate cancer confined to the pelvis, long-term results and prognostic factors were analyzed. During the period 1975-1989, 44 cases were treated with staging pelvic lymphadenectomy followed by monotherapy using external beam irradiation by Linac X-ray and/or fast neutrons and observed without any treatment until relapse was evident. All patients were followed until death or for a mean of 78.6 mo (range: 36-113 mo) for patients still alive. Four cases died of prostate cancer at 26, 28, 54, and 83 mo from the start of radiation. Eleven cases died of other causes (10-72 mo, mean 36.4 mo). Fourteen cases (31.8%) manifested clinical relapse of cancer; 4 had local relapse, 7 developed bone metastases, and 3 relapsed at lymph nodes. After relapse, endocrine therapy was effective in most cases. The five-year disease-free survival rates of pN0 (32 cases) and pN1 (8 cases) patients were 79.8% and 52.5%, respectively, but that of pN2 (4 cases) was worse. Cause-specific survival was similar between patients with pN0 and pN1 disease, the rate at 5 yr being 92.5% in the former and 100% in the latter. Those with high levels of serum prostate specific antigen (PSA) before treatment and advanced local disease (clinical stage C) showed unfavorable prognoses. The number of argyrophilic nucleolar organizer regions (AgNORs) might be a predictive factor in patients treated with irradiation. In conclusion, prostate cancer patients with stage A2-C, diagnosed as pN0-1 by staging pelvic lymphadenectomy, were successfully treated with external beam radiotherapy.(ABSTRACT TRUNCATED AT 250 WORDS)