Objectives. To determine the rate of loss of potency after radiation therapy (RT). Methods. Two hundred ninety men with localized prostate cancer were evaluated prospectively before and after RT to the prostate alone for change in erectile function. Data were collected before treatment by way of a questionnaire using a simple three-tier potency scale and after RT by the clinician at each follow-up visit. Results. At 12 months, 62% of men (90 of 146) who were potent before RT preserved their potency; at 24 months, this figure was 41%. Men who had 'normal' potency before RT were statistically significantly more likely to remain potent after RT. Conclusions. We believe that detailed knowledge of potency rates before and after RT is important for current decision-making and for evaluating new treatment techniques.