PURPOSE:We aimed to assess objectively the integrity of the parasympathetic neural pathway that controls the inflow choke vessels to the corpora cavernosa in a group of male patients with postproctectomy erectile dysfunction.METHODS:The study group was male patients with erectile dysfunction after proctectomy for rectal cancer and inflammatory bowel disease identified by sexual function questionnaire. The group underwent two consecutive nights of home nocturnal penile tumescence monitoring with the Nocturnal Electrobioimpedance Volumetric Assessment device. The control group was also monitored. It comprised preoperative potent patients with rectal cancer and inflammatory bowel disease who had not yet undergone a variety of surgical procedures. Demographics and nocturnal penile tumescence parameters were recorded, including number, duration, and percentage increase in penile volume of tumescent events.RESULTS:Thirty‐four impotent study group and 28 potent control group patients underwent nocturnal penile tumescence monitoring. The groups were well matched for mean age (difference, 1.4 years; 95 percent confidence interval, −5.8 to 8.6 years) and proportion with rectal cancer (difference, 6 percent; 95 percent confidence interval, −1 to 13 percent). The number of nocturnal penile tumescent events was greater for the potent group than for the control group (mean rank, 40.4vs. 24.2; P = 0.0004). There was no significant difference between the mean duration (difference, 2.6 minutes; mean rank, 27.9vs. 34.4;P= 0.16) or the mean penile volume increase (difference, 5.4 percent increase; mean rank, 30.6vs. 32.6;P= 0.66) for tumescent events between the study and control groups. Mean age was significantly higher in complete than in partial impotence (60.9vs. 53.1 years; difference, 7.8 years; 95 percent confidence interval, 0.1 to 15.5 years). There was a nonsignificant trend to a lower mean number of tumescence events among sildenafil responders than among nonresponders (3.5vs. 4.8 events; mean rank, 11.2vs. 17.3;P= 0.14).CONCLUSION:Nocturnal penile tumescence activity is diminished but not ablated by the trauma of surgical dissection. This suggests that some of the cavernous nerves that govern inflow to the corpora cavernosa are intact after surgery and that the nerve lesion responsible for erectile dysfunction is partial, and it explains why the response to sildenafil in such patients is surprisingly high.