Several recent reports have indicated an increased prevalence of gallstones in association with pregnancy. If these reports are true, the early puerperium should be a favorable time to detect the disease in its initial stages and follow its natural course. Accordingly, the gallbladder was examined by ultrasound in 980 women during the immediate postpartum period and in 150 nulliparous, age-matched healthy volunteers. Gallstones were detected in 12.2% of the puerperal women and in 1.3% of the control group. In 70 patients who had stones in a functioning gallbladder, 22 (31%) had had attacks of biliary colic. The history of pain was more common in patients with stones greater than 10 mm in diameter. Forty-one women with small stones (< 10 mm) were followed clinically and ultrasonographically for between 6 and 24 (mean = 8.7) mo. All remained pain-free, and in twelve subjects (29%) the stones disappeared. Gallbladder bile was examined in 11 normal volunteers (controls) immediately after delivery and in 19 women with small stones 39 +/- 6 days postpartum. Bile was saturated with cholesterol in the controls and was unsaturated in patients with gallstones. We conclude that in our population pregnancy is a very important pathogenetic factor favoring gallstone formation. Attacks of biliary colic appear early and frequently in young Chilean women with this disease. Unexplained disappearance of small stones frequently occurs: in some cases it is likely to be the result of spontaneous dissolution because bile becomes unsaturated within a few weeks of delivery.