Findings in two patients having brown pigment bile stones, recurrent 18 and 36 months after cholecystectomy, are reported. Present data suggest that bile infection precedes rather than follows the formation of brown stones. The present data are part of a prospective study of 600 consecutive patients who underwent operation for gallstones and in whom clinical and laboratory findings, intra- and postoperative bile culture and bile pH were related to the analysis of stone composition by X-ray diffractometry and infrared spectroscopy. Both patients during the first operation underwent cholecystectomy, sphincterotomy and T-tube drainage. Bile culture was negative both at operation and in the first 3 to 5 samples obtained from the T-tube every 2 days, while Escherichia coli was found, starting from the tenth and fourteenth postoperative days, respectively. At the second operation, typical recurrent "earthy" brown stones, easily crushed with the fingers, with no central nucleus of a different structure were found. Previous sphincterotomy became stenotic, and E. coli was found in the operative bile in a concentration higher than 10(6) per ml. It is suggested that bile infection by E. coli, in addition to bile stasis, plays a crucial role in the pathogenesis of brown pigment stones.