The advent of bile acid therapy has shed some light on the mechanisms involved in determining bile lipid secretion. The administration of cholelytic bile acids results in a lowering of cholesterol percent molar and saturation index due to a reduction in cholesterol secretion. Studies carried out after administration of bile acids showed initially that biliary cholesterol secretion rates were dependent on the hydrophobic/hydrophilic balance of the prevailing bile acid present in bile. However, more detailed investigations showed that some bile acids (cholic and chenodeoxycholic acids) did not follow this rule because of the presence of other mechanisms involved in determining biliary cholesterol secretion and a possible link between cholesterol synthesis and biliary cholesterol secretion. Several different human models have been used in more recent studies to arrive at a better understanding of the mechanisms involved in determining bile lipid secretion: obese patients, obese patients in rapid weight loss, patients with non-familial hypercholesterolemia and primary biliary cirrhosis. The findings in these studies indicate how modifications in biliary lipid secretion can easily be induced when there are changes in the relative amounts of bile acids. These changes may bring about modifications in intestinal absorption, liver synthesis, and secretion of cholesterol and bile acids that could possibly lead to the formation of lithogenic bile and subsequently to cholesterol gallstones.