Abstract Background Stapled haemorrhoidectomy is gaining wide acceptance but there is still some concern about the risk of injury to the internal anal sphincter (IAS). IAS function and morphology, and anal canal sensitivity were studied prospectively in patients undergoing this operation. Methods Twenty patients (11 women; mean age 43 years) with stage III haemorrhoids entered the study. All underwent preoperative anorectal manometry, rectoanal inhibitory reflex (RAIR) testing and three-dimensional transanal ultrasonography. A test of anal sensation was administered to evaluate ability to discriminate between air and warm water. All the investigations were repeated 6 months after the operation. Results The mean(s.d.) maximal resting pressure was 87(30) mmHg before surgery and 81(20) mmHg afterwards (P not significant). The maximal squeeze pressure did not change after operation (178(43) versus 174(60) mmHg). The RAIR showed the same features in 19 of 20 patients before and 18 of 20 after operation. Three-dimensional ultrasonography demonstrated no changes in the width of the IAS (mean(s.d.) 2·1(4) mm before and 2·1(3) mm after surgery). The ability of the anal mucosa to discriminate air from warm water improved in five patients. Continence scores did not differ significantly after 6 months. Conclusion Stapled haemorrhoidectomy does not affect the function and morphology of the IAS in the long term. The sensitivity of the anal canal can improve in patients with preoperative sensory impairment.