['To assess the association between obstructive sleep apnea syndrome (OSAS) and erectile dysfunction (ED). OSAS is associated with fatigue and daily dysfunction. We hypothesized that OSA severity will correlate with ED. We also sought to determine the specific features with predictive value for ED in patients with OSAS.', 'A total of 209 patients evaluated for potential OSAS were given a sleep questionnaire, a revised International Index of Erectile Function questionnaire (composed of three main topics: sexual satisfaction, erection during intercourse, and morning erection), and then underwent a full-night in-laboratory polysomnography examination. The data were scored blindly.', 'A decrease in erectile function was found in all OSAS groups compared with those without OSAS. However, the only group for whom this reduction was statistically significant (P <0.05) in all ED dimensions was the severe OSAS group (respiratory disturbance index [RDI] greater than 40). Furthermore, the patients complaining of severe ED had the greatest RDI. Stepwise multiple logistic regression analyses revealed that the predicting factors for ED in OSAS patients were age, morning tiredness, and RDI.'] Is severe , but not mild , obstructive sleep apnea syndrome associated with erectile dysfunction?