Resting anal canal pressure was measured in 15 patients with anal fissure before and after lateral internal sphincterotomy. This pressure was found to be significantly higher in these subjects (mean 95 +/- 23.08 mm HG) than in the control group (mean 66.10 +/- 14.28 mm Hg) before surgery (P less than 0.005). After surgery, a normal anal canal pressure was produced. The authors maintain that anal canal spasm is responsible for chronicity of anal fissures.