Many cases of gallstones can be explained in terms of the established risk factors, especially obesity. However, gallstones develop in some women who are not obese, and the causes are unknown. Biochemical studies have shown that slow intestinal transit is associated with lithogenic bile. We have tested the hypothesis that intestinal transit is abnormally slow in normal-weight women with gallstones. In a population survey, 1058 women aged 25-69 years, registered with general practitioners in Bristol, UK, underwent cholecystosonography. Gallstones were identified in 48 women, of whom 15 were of normal weight (body mass index < or = 25 kg/m2). These women and age-matched controls with healthy gallbladders then underwent measurement of whole-gut transit time (WGTT); the measurement was done directly when possible, or calculated from records of three defaecations. The mean WGTT was significantly longer in the women with gallstones than in the controls (82 vs 63 h; mean difference 19, 95% CI 2-37 h). Stool output was also lower in the women with gallstones (74 [SD 54] vs 141  g per 24 h, p = 0.015). There was no significant difference between cases and controls in body mass index, waist-hip circumference ratio, parity, plasma triglyceride concentration, or alcohol intake. Normal-weight women with gallstones tend to have slow intestinal transit and this feature could explain why they have gallstones.