Prostate cancer has become the most frequent malignancy in older men in Western countries. Radiotherapy is a common treatment modality for early stage prostate cancer. 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. Oral drug therapy represents the first-line management option for patients with ED. The results of the studies completed thus far with sildenafil show that a phosphodiesterase type 5 inhibitor is a safe and efficacious drug for the treatment of post-radiation ED. No studies have investigated the efficacy of tadalafil in men complaining of ED after radiotherapy of prostate cancer. Because of the extended period of effectiveness, which may last up to 36 hours after intake, tadalafil allows freedom in the choice for time of sexual activity. Also the absence of restrictions of tadalafil intake with food or alcohol should simplify its administration. This randomized, double-blind, placebo-controlled, cross-over study has been designed to evaluate the efficacy and safety of “on demand” dosing of 20-mg of tadalafil or placebo administered for 12 weeks to patients with ED after external-beam radiotherapy for prostate cancer.