Intestinal bypass operation for obesity results in substantial weight loss only if the small bowel segment left in function is 50 cm or less. The anatomical changes induce interruption of the enterohepatic circulation of bile acids, which result in bile acid malabsorption. This review discusses the various aspects of the disturbed bile acid metabolism. A small number of controlled prospective studies have focused on the problems of the jejunoileal ratio (JIR) of the functioning segment in relation to the changes induced on the bile acid metabolism. 1:3 JIR results in a significantly: (1) lower bile acid pool size; (2) lower postprandial concentration of bile acid in the jejunum: (3) lower ratio of glycine to taurine conjugates; (4) higher cholesterol saturation index in bile, compared to 3:1 JIR. Thus, the studies mentioned have not only elucidated the changes in bile acid metabolism after jejunoileostomy, but also given support to a new hypothesis that a functioning upper jejunum is necessary for the bile acid synthesis as such. This hypothesis is further supported by the finding that 1:3 JIR at follow-up has a three fold higher rate of gallstones than 3:1 JIR (p less than 0.05).