Aim The aim of this prospective study was to clarify the frequency of male sexual dysfunction after laparoscopic total mesorectal excision (LTME) and to examine the relationship between pelvic autonomic nerve (PAN) preservation status and functional outcomes. Method Candidates for LTME were included in this study. PAN preservation status after LTME was examined in detail by video review. Patients completed a functional questionnaire (the International Index of Erectile Function) before and 3, 6 and 12 months after the operation. Results Twenty-six patients who underwent LTME were assessable. Detailed video reviews identified inadvertent PAN damage during surgery. PAN injury was observed in 11 cases (41%), including eight cases (32%) of inadvertent PAN damage (incomplete preservation group). There was a trend toward increasing inadvertent PAN injury rate in patients with high body mass index and large tumours. The results from all patients who underwent LTME showed no deterioration in total International Index of Erectile Function or its domain scores 12 months after surgery. In the incomplete preservation group, these scores temporarily decreased (3 and 6 months after surgery), but such deterioration was not observed in the complete preservation group. Most of the 12 patients with potentially active erectile function before the operation recovered this function, and only one patient (7%) with PAN injury was still judged as inactive 12 months after surgery. Conclusion The proportion of patients with sexual dysfunction after LTME is low. With the enhanced visibility of the laparoscope, inadvertent PAN injury was detected in a significant number of cases and associated with transient deterioration of sexual function.