To determine whether highly saturated bile is a congenital or acquired characteristic of Pima Indians and to elucidate the basis of the rapid postpubertal increase in gallstones in Pimas, we studied the bile of 66 Pimas nine to 21 years of age. Highly saturated bile is not prevalent among Pimas under the age of 13, but bile saturation increases significantly (P less than 0.05) in both sexes during pubertal growth and development. Bile saturation was 15 per cent higher in females than males. Bile acid pools increased with age in the young men, but not in women. Bile cholesterol saturation correlated with obesity (r = 0.41; P less than 0.001) and urinary estrogen excretion (r = 0.44; P less than 0.001). Highly saturated bile may be present for several years before the onset of cholesterol cholelithiasis.