To assess gallbladder function, sonographic gallbladder volume determinations have been used with increasing frequency. This study presents a modified and automated procedure for gallbladder volume determinations using Simpson's rule of integration, Simpson's method. This method is a standard option in the data systems of many sonographic instruments. Simpson's method was validated in vitro and in vivo, and it was compared with the sum-of-cylinders method for gallbladder volume determinations. In vitro assessment indicated that the two methods were equally accurate, with Simpson's method being more precise. The absolute deviation was independent of the size of the volume and of the shape of the gallbladder. In vivo Simpson's method was validated on 11 patients with cholecystitis. The gallbladder volumes (mean 65 mL; Range 20 mL to 130 mL) measured by sonography differed from the aspirated volumes by 1.5 mL (SD 10.4 mL). Thus Simpson's method is an accurate, precise, and fast method for sonographic gallbladder volume determination.