SUMMARY In two groups of gallstone patients ideally suited for medical treatment, the effect of six to 18 months' therapy was compared retrospectively in 52 given chenodeoxycholic acid (CDCA) and 46 given ursodeoxycholic acid (UDCA). The minimum dose (mg kg-1day-1) 18 months' treatment with CDCA and UDCA in diminishing the saturation of bile with cholesterol and in dissolving gallstones. We also compared the frequency and severity of symptons (such as biliary colic and non-specific dyspepsia) and the incidence of side-effects, before and during treatment. This paper reports our findings. Methods PATIENTSFifty-two patients treated with CDCA and 46 treated with UDCA fulfilled the criteria listed above and were selected for the comparative study. Details of their ages, sex ratios, and body weights are summarised in Table 1. There were no significant differences between the groups for any of these