['Recently it has been reported that bacterial DNA has been detected in mixed cholesterol stones (cholesterol content < 95%), which were not previously believed to be related to bacteria, using the polymerase chain reaction (PCR). We examined bacterial DNA in pure cholesterol stones to clarify the mechanism of initiation or promotion of the formation of cholesterol gallstones.', 'We examined 69 gallstones (30 brown pigment stones, 21 pure cholesterol stones, and 18 mixed cholesterol stones). Bacterial DNA was extracted from the core of the gallstones and amplified by PCR. Bacteria species in gallstones were identified by DNA sequencing of the PCR products.', 'Bacterial DNA was detected in 26/30 brown pigment stones (87%), in 12/21 pure cholesterol stones (57%) (cholesterol content = 100%), and in 12/18 mixed cholesterol stones (67%) (cholesterol content = 82-95%). Bacterial species in gallstones were identified by DNA sequencing of PCR products. Eighty percent of bacteria in brown pigment stones were gram-negative rods or anaerobes. In contrast, 100% of bacteria in pure cholesterol stones were gram-positive cocci. The bacteria in mixed cholesterol stones consisted of 40% gram-positive cocci, 50% gram-negative rods, and 10% anaerobes.'] Are gram-positive cocci associated with the formation of completely pure cholesterol stones?