The role of bacteria in pigment gallstone disease. One hundred ten of nine hundred sixty consecutive patients who underwent surgery for gallstones (GS) had pigment stones (PS) (11.45%). Fifty brown PSs contained calcium bilirubinate, small amounts of cholesterol, and always calcium palmitate, were usually found in the common duct (96%), and were almost always associated with bile infection (98%) and diffuse erosion of the biliary mucosa. Fifty-one black PSs contained bilirubin polymers, calcium carbonate, and/or phosphate, seldom cholesterol, and never evident amounts of calcium palmitate, were mostly found in the gallbladder, and were associated with hemolysis or liver damage and with hyperplastic cholecystosis. Bile infection was found in 19.6% of cases, but bacteria were never found in the center of black PSs by scanning electron microscopy. Nine additional patients (8.2% of PSs, 0.9% of GSs) had concomitant black and brown PSs that were mostly found in the common duct and were always associated with bile infection. It is suggested that, even if PSs with concomitant black and brown material can be found, black and brown PSs greatly differ not only in pathogenesis but also in clinical behavior and treatment. In particular bacterial infection is important only in the pathogenesis of brown PSs while it plays no role in the initial formation of cholesterol, mixed or black GSs. 