Total colectomy with ileo-anal anastomosis is an effective treatment for ulcerative colitis and familial adenomatous polyposis. The absence of the colon and the coexistence of bile acid malabsorption may increase bile lithogenicity, but data on biliary lipid composition in patients with this operation is lacking. Our aim was to assess bile lithogenicity, bile composition and mass of biliary lipids within the gallbladder. We studied 11 patients with total colectomy and ileo-anal anastomosis and 16 healthy controls. We measured the percentage composition of conjugated bile acids and the masses within the gallbladder of the three main biliary lipids. This method, in contrast with measurement of cholesterol saturation index, can determine the cause of bile lithogenicity in terms of absolute modifications of the biliary lipids. There was no difference in the cholesterol saturation index between patients and controls. Colectomy patients had reduced masses of all three biliary lipids (medians and ranges, mmol): cholesterol 0.11 (0.03-0.24) vs. 0.36 (0.02-0.96), P < 0.02; bile acid 1.62 (0.75-5.21) vs. 3.95 (1.27-8.70), P < 0.01; phospholipids 0.35 (0.07-0.69) vs. 1.14 (0.14-3.00), P < 0.002. They also had reduced per cent deoxycholic acid: 3.8 (0.0-27.6) vs. 17.4 (6.4-44.7), P < 0.005, and increased percent cholic acid: 44.9 (23.3-71.4) vs. 34.3 (19.2-57.9), P < 0.05. We conclude that, despite having bile acid malabsorption, patients with colectomy and ileo-anal anastomosis have a normal cholesterol saturation index, caused by a concomitant reduction in the masses of all three biliary lipids. The reduced per cent biliary deoxycholic acid may help explain the reduced cholesterol and phospholipid masses in these patients. Total colectomy with ileo-anal anastomosis does not seem to predispose to the formation of cholesterol gallstones.