The purpose of this study was to develop a method by which ascitic volume can be calculated using transcorporeal ultrasonography, and to determine the accuracy of this method by comparison with the volume of distribution of a radiolabeled tracer (indicator dilution technique [IDT]). Subjects with ascites confirmed by ultrasonography were recruited from the San Francisco General Hospital Gastroenterology and Liver Clinics. With subjects in the prone position on their hands and knees, ultrasonographic measurements were obtained along the ventral surface of the abdomen. The greatest vertical depth of ascitic fluid was recorded, and the abdominal circumference was measured from this point. The ascitic fluid volume was modeled as a segment of a sphere. IDT was performed as the reference method by injecting 99mTc-labeled macroalbumin into the peritoneal cavity and determining the volume of distribution of the indicator. Nine patients were evaluated. The median volume of ascites measured by the IDT was 11.2 L (range, 1.5-17.0 L). The median volume calculated by the ultrasonographic method was 10.3 L (range, 1.2-18.0 L). The correlation coefficient between the ultrasonographic and IDT was 0.96 (P < .001). Our technique accurately determines the volume of ascites using simple ultrasonographic measurements.