A prospective, controlled study was carried out to determine if there was an association between hiatus hernia and cholelithiasis. The prevalence of gallstones was calculated in 100 patients (50 men and 50 women) with radiological evidence of hiatus hernia. 100 subjects (50 men and 50 women) without hiatus hernia, matched for age, body-weight, and number of pregnancies, acted as controls. Gallstones were twice as common in the former group (34%) as in the control group (17%) (P less than 0.01). Moreover, a comparison of the lipid composition of the gallbladder bile in 15 subjects operated on for hiatus hernia with that in 17 patients operated on for another digestive disease showed that the molar percentage of choelsterol in the bile and the cholesterol saturation index were significantly higher in the patients with hiatus hernia (9.3 +/- 1.1, mean +/- S.E.M., 1.41) than in the control subjects (5.9 +/- 0.5; 0.97) (P less than 0.01). These results suggest an association between hiatus hernia and cholelithiasis. It is likely that common factors, probably related to a low-residue diet, predispose to the two diseases.