A prospective, manometric trial of anal fissure treated by subcutaneous lateral internal sphincterotomy (SLIS) was designed to elucidate the pathophysiology of this condition. Anorectal manometry with a closed, precalibrated, water-filled microballoon using the station pull-through technique was performed on 13 patients with anal fissure before, and at one and 150 days after SLIS. The results were compared with 13 control subjects, matched for age and sex, who had no history of anal disease. Both resting pressure (RP) and maximum voluntary contraction pressure (MVCP) were measured at centimeter intervals of the anal canal. At all levels RP was significantly higher in the preoperative patients compared with controls (p less than 0.0001). After operation RP fell significantly at all levels with the result that there was no significant difference in RP between postoperative patients and controls, except at 4 cm from the anal verge, where there remained a significant elevation in RP in the postoperative group. There was no significant difference in the two sets of postoperative manometric results. All patients underwent rapid healing and resolution of their symptoms. MVCP did not change significantly after operation, nor did it differ from the control values. This suggests that the increase in RP is due to activity of the internal anal sphincter. This over-activity is present throughout the entire length of the internal anal sphincter and sphincterotomy of its lowest portion returns RP to normal values throughout most of the anal canal.