The aim of this study was to investigate frequency and degree of ED in patients with severe sleep apnea and to evaluate the results of only continuous positive airway pressure (CPAP) therapy on ED in patients with severe obstructive sleep apnea syndrome (OSAS). This was a prospective clinical trial study. Patients with severe sleep apnea (40) were randomized into two groups. Multiple questionnaire investigation and laboratory evaluation were performed for ED, severity of OSAS and psychological status. Group 1 was treated with CPAP and group 2 was treated with only antidepressant medication for at least 1 month. Patients were evaluated after 1 month of therapy and compared with initial ones. Before CPAP, the International Index of Erectile Function (IIEF)-5 scores were significantly correlated only with body mass index (BMI; P=0.007) and not correlated significantly with Epworth Sleepiness Scale scores (P=0.286), lowest SaO(2) (P=0.182), Beck's Depression Inventory scores (P=0.302) and apnea/hypopnea index (P=0.279). After 1 month of regular CPAP usage, mean value of IIEF-5 score was 15.71+/-5.12 before CPAP and were improved up to 19.06+/-3.94, statistically significant. All subjects responded positively to the CPAP treatment and their erection status was improved positively. We have found a correlation between severe OSAS and ED. CPAP is effective in improvement of sexual performance of these patients.