Abstract The activity of the internal anal sphincter in patients with fissure has been studied by measuring anal pressure and motility with a small balloon probe. The mean maximum anal pressure in 12 patients with fissure (116·8 ± 21·8, s.d., cm H2O) was significantly higher lhan that in 40 control subjects (85·0 ± 20·5 cm H2O; P < 0·01). Ultra-slow pressure waves were present in 80 per cent of the patients and in 5 per cent of the controls. Both the high pressure and ultra-slow waves were due to abnormal activity of the internal sphincter. This could be corrected equally well by either dilatation or lateral subcutaneous sphincterotomy.