In order to determine the influence of pelvic radiotherapy on potency, a questionnaire has been sent to all living patients who had been treated for a prostate carcinoma with exclusive external beam radiotherapy from 1980 to 1991 in Centre Hospitalier Lyon Sud. One hundred and six patients have been asked about the frequency of intercourse prior to and after treatment, and about possible modifications of their sexual activity after radiotherapy. At the time of the study, median follow-up was of 47 months. 5 and 10 years survival rates were respectively 76 and 53% for overall survival, and 65 and 33% for disease-free survival. Severe complications rate related to radiotherapy was 2%. Among the 56 patients who returned a completed questionnaire, 12 (21%) were impotent prior to treatment. During the first year after radiotherapy, 10 patients were given an hormonal treatment and became impotent. Among the other 34 evaluable patients, 20 (59%) patients were still potent one year after radiotherapy, and sexual activity was unchanged for 3 patients (15%), slightly altered for 11 patients (55%) and altered for 6 patients (20%). Erection capacity was modified for 82% of the patients, ejaculation for 59%, orgasm for 47% and libido for 6%. In order to determine prognostic factors of conservation of sexual activity, we analyzed the distribution of potent and impotent patients according to age, tumor stage, Gleason score, frequency of intercourse prior to treatment and size of the radiation fields. Only age < or = 64 was associated with a higher percentage of preservation of potency: 76% versus 59% in patients older than 64 (p < 0.05).(ABSTRACT TRUNCATED AT 250 WORDS)