During treatment with ursodeoxycholic acid (UDCA), the fasting gallbladder volume increases by a yet unknown mechanism. The present study tests whether in vitro human gallbladder contractility in response to acetylcholine and cholecystokinin is affected by UDCA therapy. Gallbladder tissue was obtained from 15 patients treated with UDCA (10 mg/kg/day) during three weeks prior to surgery, and from 15 comparable patients not treated. Data were correlated with in vivo contractility, bile composition, and gallbladder wall inflammation. The inflammation score was lower in the treated patient group. UDCA treatment enhanced gallbladder contractility in vitro: Dose-response curves for acetylcholine and cholecystokinin were both shifted to the left, and the maximal contractile stress generated in response to cholecystokinin was higher in the treated group, whereas the maximal acetylcholine-induced stress was not increased. Maximal cholecystokinin-induced stress correlated positively with fasting gallbladder volume and negatively with the biliary cholesterol saturation index, but not with bile salt hydrophobicity or gallbladder wall inflammation score. In conclusion, UDCA treatment improves in vitro gallbladder contractility, possibly related to a reduced biliary cholesterol saturation. Increased fasting gallbladder volumes during UDCA treatment thus do not appear to result from decreased gallbladder muscle contractile strength.