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 than that in 40 control subjects (85-0 +/- 20-5 cm H2O; P less than 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 of 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.