The present study aims to prospectively compare the sexual function in males before and after vasectomy surgery using the international index of erectile function (IIEF). From October to December 2002, sixty-four patients who were candidates for male sterilization in the vasectomy program of the Urology Section at the General Hospital of the University of São Paulo were included. The same investigator applied the IIEF before and 90 days after the surgery. The mean scores obtained on pre and postoperative visits for all domains of sexual function were analyzed and compared with the Wilcoxon test. The mean patient age was 35 years (range from 25 to 48 years) and the mean number of children per man was 3. The total mean score of the IIEF was 64.06 before surgery and 65.64 after the procedure, with this difference considered statistically significant (p < 0.001). Sixty-seven per cent of the patients improved their scores, versus 17% and 16% who showed worsening or no change at all in IIEF scores following surgery, respectively. Of the 5 sexual function domains, desire and sexual satisfaction presented statistically significant improvement. This study showed that vasectomy caused a positive impact on sexual function, especially on desire and sexual satisfaction, in the majority of men undergoing surgery. There was no case of surgery-related erectile dysfunction.