PURPOSE:This study was designed to investigate the effect of lateral sphincterotomy on internal anal sphincter function in patients with chronic anal fssure.METHODS:Using an eight‐channel perfusion catheter and computerized data analysis, a prospective manometric study was performed on patients with chronic anal fissure undergoing lateral sphincterotomy (LS).RESULTS:Mean resting pressure (MRP) in patients with anal fissure (85.1 mmHg) was significantly higher(P=0.012) than control subjects (63.3 mmHg). One week following LS there was a significant reduction in MRP (50.0 mmHg;P=0.0014), and this was maintained when reassessed five weeks later (MRP=56.4 mmHg;P=0.0019). There was no significant difference in coefficent of variation (a measure of the degree of manometric asymmetry of the anal canal) in the control group (mean, 8.9 percent) and in patients with anal fissure (mean, 7.7 percent;P=0.43). LS created a significant increase in anal canal resting manometric asymmetry when assessed at one (mean, 17.3 percent;P=0.0013) and six weeks (mean, 11.7 percent;P=0.027) after the procedure.CONCLUSION:LS produces a global and symmetric decrease in anal canal resting pressure. In addition, it produces a significant increase in manometric asymmetry of the resting anal canal by creating a detectable segmental defect.