Because of the high incidence of post-radiation erectile dysfunction (ED), up to 72% after external-beam radiotherapy, this patient category represents a most difficult therapeutic challenge. Therefore, prevention of ED could be more effective than treatment. Tadalafil, a new phosphodiesterase type 5 inhibitor, has been recently introduced. No studies have investigated the efficacy of tadalafil in preventing ED in patients undergoing radiotherapy for prostate cancer. Efficacy of tadalafil can last up to 36 hours after intake. This will result in a prolonged and continuos enhancement of penile vascular responsiveness. This randomized, double-blind, placebo-controlled study has been designed to evaluate the efficacy of 20-mg of tadalafil administered for 12 months in maintaining erectile function of potent patients undergoing external-beam radiotherapy for prostate cancer.