Bacteria are traditionally accorded a greater role in pigment gallstone formation in Eastern populations. Stone color is thought to predict the presence of bacteria; that is, black stones (Western predominant) are supposedly sterile and brown stones (Eastern predominant) contain bacteria. We previously reported that, regardless of appearance, most pigment gallstones contain bacteria. This study examined, in a large Western population (370 patients), the incidence, appearance, and chemical composition of pigment stones, and the characteristics of gallstone bacteria. One hundred eighty-six pigment stones were obtained aseptically. Bacteria were detected by means of scanning electron microscopy and gallstone culture. Chemical composition was determined by infrared spectroscopy. Bacteria were tested for slime and β-glucuronidase production. Seventy-three percent of pigment stones contained bacteria. Choledocholithiasis was associated with gallstone bacteria. Ca-bilirubinate was present in all pigment stones. Capalmitate was characteristic of infected stones, and more than 75% Ca-carbonate was characteristic of sterile stones. Neither chemical composition nor stone appearance predicted the presence of bacteria. Ninety-five percent and 67% of infected pigment stones contained bacteria that produced slime and β-glucuronidase, respectively. Most pigment stones contained bacteria that produced β-glucuronidase, slime, and phospholipase, factors that facilitate stone formation. Thus bacteria have a major role in Western pigment gallstone formation. Furthermore, gallstone color did not predict composition or bacterial presence.