Endoscopic sphincterotomy (ES) alters the structure and motor function of the sphincter of Oddi (SO). The magnitude and duration of these changes, however, have not been critically examined. Before ES, 22 patients with common bile duct stones were evaluated by endoscopic retrograde cholangiography. The pressure gradient between the common bile duct and the duodenum, the SO basal pressure, and the SO peak phasic pressures were obtained. After ES, the electrosurgical incision length was determined using the extended papillotome and an inflated Fogarty balloon as reference. A high correlation existed between the endoscopist's estimate of ES incision size using this technique and the actual length of simulated incisions fashioned in cardboard mounts. These studies were repeated in all 22 patients at 1-yr follow-up and in 8 of these patients at 2-yr follow-up. At 12 mo and 24 mo after ES, the common bile duct (CBD) to duodenal pressure gradient and the sphincter of Oddi basal pressure were virtually eliminated. The amplitude of SO phasic contractions was significantly diminished 12 mo after ES (124 ± 16 mmHg to 37 ± 10 mmHg; p < 0.001), but 24 mo after ES, SO phasic contraction amplitude was not significantly different from the values before ES. Incision length at 1-yr follow-up was reduced in the group of 22 patients from 11.6 ± 0.8 mm to 8.3 ± 0.5 mm (p < 0.001), and in the group of 8 patients from 11.0 ± 1.5 mm to 7.5 ± 0.7 mm (p < 0.025). After an additional 12 mo, however, i.e., 24 mo after ES, the incision length was 6.5 ± 0.7 mm. There was no significant difference in incision length between the 12-mo and 24-mo examinations. We conclude that after ES, incision length decreases during the first year. There appears to be no further significant reduction in incision length at 2 yr. In addition, the reduction of the CBD to duodenal pressure gradient and the SO basal pressure remain unchanged for at least 2 yr. These manometric findings support the observation that after ES the enlarged opening of the CBD into the duodenum remains open for at least 2 yr.