PURPOSE:Controlled trials have demonstrated the efficacy of sildenafil for “mixed etiology” erectile dysfunction, but this may not be the case if there is underlying pelvic parasympathetic nerve damage. We aimed to determine the efficacy of sildenafil after rectal excision for rectal cancer and inflammatory bowel disease.METHODS:Patients with erectile dysfunction after rectal excision were randomly assigned in a double‐blind manner to sildenafil or placebo groups. After unblinding, placebo patients crossed over to open sildenafil. Primary end points were improvement in erectile function on a global efficacy question and erectile function questionnaire scores. Secondary end points were frequency and severity of side effects.RESULTS:Thirty‐two patients were randomly assigned, and two dropped out before randomization. Fourteen received sildenafil, and 18 received placebo. Eleven (79 percent) of 14 responded to sildenafil, on global efficacy assessment, compared with 3 (17 percent) of 18 taking placebo (mean difference, 61.9 percent; 95 percent confidence interval, 34.4 to 89.4 percent;P= 0.0009). Sildenafil improved both erectile function domain scores (mean difference, 13.3; 95 percent confidence interval, 7.9 to 18.7;P= 0.0001) and total International Index of Erectile Function scores (mean difference, 30.6; 95 percent confidence interval, 18.7 to 42.6;P< 0.0001) from pretreatment baseline scores. Placebo did not produce improvement in either erectile function (mean difference, 1.7; 95 percent confidence interval, −0.8 to 4.2;P= 0.16) or total International Index of Erectile Function scores (mean difference, 5; 95 percent confidence interval, −1.1 to 11.1;P= 0.1). Ten (100 percent) of 10 crossover patients not responding to placebo did respond to sildenafil (difference, 100 percent;P< 0.0001). Sildenafil improved both erectile function domain scores (mean difference, 16.8; 95 percent confidence interval, 9.7 to 24;P= 0.002) and total International Index of Erectile Function scores (mean difference, 29.5; 95 percent confidence interval, 15.8 to 43.2;P= 0.003) from precrossover baseline scores. Seven (50 percent) of 14 patients on sildenafil compared with 4 (22 percent) of 18 on placebo experienced side effects (difference, 28 percent; 95 percent confidence interval, −4.4 to 60.4 percent;P= 0.14), 91 percent of which were mild and well tolerated.CONCLUSION:Sildenafil completely reverses or satisfactorily improves postproctectomy erectile dysfunction in 79 percent of patients. Side effects are usually mild and well tolerated. The damage incurred by the pelvic nerves after proctectomy, less profound than after prostatectomy, is likely to result in a partial parasympathetic nerve lesion.