Measurements of biliary lipid secretion rates were performed in 14 non-obese patients with radiolucent gallstones (9 females, 5 males; mean age 48 years; mean body weight 65 kg) and in 14 healthy male volunteers (mean age 26 years, mean body weight 74 kg). The results in the gallstone patients differ in several respect from those obtained in the volunteers. Molar percentage of cholesterol was higher (5.8 versus 5.0 mol%; P less than 0.05) and molar percentage of bile acids lower (73.8 versus 76.9 mol%; P less than 0.05) in the gallstone patients. However, these changes were not followed by notable differences in cholesterol saturation of bile (94% vs 88%). Generally, hepatic secretion rates of cholesterol were significantly elevated in the gallstone patients (55 vs 46 mg/h; P less than 0.05) whereas outputs of bile acids and phospholipids did not differ between the two groups. Although patients with cholesterol gallstones tended to have a lower percentage of chenodeoxycholic acid (38 versus 42 mol%) and increased deoxycholic acid (23 versus 16 mol%) in their bile, these differences were not significant. Nevertheless, in patients with cholesterol gallstones a significant positive correlation between deoxycholic acid secretion and cholesterol output was observed. For the whole group of patients and volunteers a positive correlation between age and cholesterol secretion could be demonstrated. The higher hepatic cholesterol secretion in gallstone patients seems not be due to differences in body weight, but rather to the older age of the patients. These results suggest that age itself or age-related changes in deoxycholic acid metabolism contributes to biliary cholesterol output in non-obese patients with cholesterol gallstones.