Abstract Anorectal myectomy has been evaluated in 29 patients with chronic constipation associated with outlet obstruction. Eight patients had histological evidence of aganglionosis. No patient was able to defaecate spontaneously more than once a week before operation. Eighteen (62 per cent) patients were able to defaecate spontaneously more than 3 times a week after operation (good results) whereas 11 patients were not. There was a significant fall in maximum resting anal pressure after operation in the patients having a good result (102·7±16·7 to 79·5 ± 27·8 cm H2O) whereas there was no significant fall in those having a poor result (118·4±25·0 to 108·8±28·5 cm H2O). The operation had no influence on squeeze pressures, length of the high pressure zone, rectal sensation or the anorectal angle. These results seem to indicate that the therapeutic effect of anorectal myectomy is merely due to reduced resting anal canal pressures.