Treatment with the somatostatin analog octreotide is associated with increased gallstone formation. The mechanism of formation of these stones is unclear. The purpose of this study was to examine the effect of a three-month treatment with octreotide on biliary lipid composition and the occurrence of cholesterol crystals in patients with acromegaly. Thirteen patients with active acromegaly, aged 24-76 years, received octreotide (100 micrograms three times daily) for three months. Fasting gallbladder bile was obtained during upper gastrointestinal endoscopy after ceruletide stimulation. Bile was studied before and at the end of the treatment period (N = 7), only before (N = 4), or only at the end of treatment (N = 2). Before treatment, all bile samples but one were supersaturated with cholesterol. However, none contained cholesterol crystals on microscopic examination. At the end of the treatment period, all but two samples were supersaturated with cholesterol. Three of nine samples contained cholesterol crystals, a proportion significantly higher than before treatment. The relative proportions of bile acids, cholesterol, and phospholipids, and the mean cholesterol saturation index were not different before and during treatment. Follow-up ultrasonography showed the occurrence of gallstones in four patients, including the three patients who had cholesterol crystals. We conclude that: (1) fasting gallbladder bile of patients with acromegaly is frequently supersaturated with cholesterol; (2) treatment with octreotide does not increase cholesterol saturation index, but may induce the occurrence of cholesterol crystals. The data are consistent with the view that gallstones induced by octreotide are cholesterol stones and suggest that the drug may impair gallbladder motility and/or decrease cholesterol nucleation time.