INTRODUCTION: Clinical research on erectile dysfunction (ED) has focused primarily on the male and the impact of treatment on their erectile function (EF) and sexual quality of life. However, ED affects the quality of life of both the male and the female partner. The literature examining the impact on the female partner resulting from treating the male's ED is somewhat limited. AIMS: To determine the efficacy of tadalafil 5 mg taken once daily compared with placebo on men's EF and sexual quality of life, and to determine the impact of this treatment on the female partner's sexual quality of life. MAIN OUTCOME MEASURES: The co-primary outcome measures for this study were changes from baseline to end point in the EF domain of the International Index of Erectile Function (IIEF), the Sexual Encounter Profile (SEP) question 2 (SEP-2) and question 3 (SEP-3), and the Sexual Quality of Life (SQoL) domain of the Sexual Life Quality Questionnaire (SLQQ) (subject and partner). Methods. Following a 4-week treatment-free run-in phase, 342 subjects and their partners were randomly assigned to either placebo (N = 78) or tadalafil 5 mg (N = 264) for 12 weeks. The subjects' and partners' responses to study measures were collected throughout the study. RESULTS: Compared with placebo, tadalafil-treated subjects showed a significant improvement on efficacy measures (P < 0.001) including changes in the IIEF-EF, SEP-2 and SEP-3. In addition, the sexual quality of life of men and their female partners, as measured by the SQoL domain, was significantly improved with tadalafil 5 mg taken once daily (P < 0.001) compared with placebo. CONCLUSIONS: Tadalafil 5 mg once daily significantly improved EF and sexual quality of life for men with ED. In addition, the sexual quality of life of the female partners of the men treated with tadalafil was significantly improved.