Udenafil is a potent phosphodiesterase type-5 inhibitor (PDE5) previously shown in studies conducted in populations of Eastern-Asian ethnicity, to significantly improve sexual function, in addition to a favourable safety profile. The purpose of this study was to evaluate the efficacy and safety of udenafil for the treatment of erectile dysfunction (ED), for the first time in a non-Eastern-Asian population. In this multicentre, randomized, double-blind, parallel, placebo-controlled study conducted in five centres in Turkey, 118 eligible subjects were randomized to receive udenafil 100mg taken as on-demand or matching placebo for an 8-week treatment period. The primary efficacy variable was the change from baseline of the International Index of Erectile Function Questionnaire-Erectile Function Domain (IIEF-EFD) score, secondary efficacy variables were changes from baseline in IIEF Questionnaire Domains' 2-5 scores (Intercourse Satisfaction, Orgasmic Function, Sexual Desire, Overall Sexual Satisfaction) and IIEF Questionnaire Grand Total score, changes from baseline in penetration success rates (SEP2) and intercourse completion rates (SEP3) and evaluation of responses to the global assessment question (GAQ). Patients treated with udenafil demonstrated significantly higher increase in the IIEF-EFD scores compared with placebo-treated subjects [4.0 (95% CI: 1.3-6.6; p=0.003)]. Similarly, greater improvements were observed in the scores for SEP2 [0.65 (95% CI: 0.02-1.3, p=0.043)], SEP3 [0.9 (95% CI: 0.3-1.5, p=0.003)] and two other IIEF Questionnaire Domains (Domain 4: Sexual Desire, Domain 5: Overall Sexual Satisfaction). The proportion of positive responses to the GAQ was greater in the udenafil compared to the placebo group (72.2% vs. 49.1%, p=0.014). The most frequent treatment-emergent adverse events were headache, flushing and rhinorrhea, all of mild or moderate severity. This is the first study to demonstrate in a non-Eastern-Asian population that udenafil 100mg taken as on-demand can effectively improve erectile function and is well tolerated.