A prospective blind study was performed to compare sonographic with postoperative findings of gallstone burden in 34 patients (10 males, 24 females; mean age 52 years). Gallstone size was assessed in single calculi (n = 15) by measuring the largest diameter by ultrasonography (US). The ellipsoid algorithm was used to estimate US volume for both single and multiple gallstones. After cholecystectomy, diameters were measured in single stones; stone volume was assessed by the ellipsoid formula (single stones) and fluid displacement (single and multiple stones). Stone volumes assessed by the ellipsoid formula and fluid displacement correlated closely (r = 0.98; p < 0.0001, n = 15). In the case of single gallstones, the mean length assessed by US was 25.3 +/- 3.2 mm and compared well with the value measured "in vitro" (24.8 +/- 3.1 mm). A highly significant and positive relationship existed between the sonographic size and the true stone size (r = 0.93; p < 0.0001, n = 15). Stone volumes assessed "in vivo" and by fluid displacement were comparable and correlated for both single (US: 6.6 +/- 1.3 vs fluid displacement: 5.8 +/- 1.4 ml; r = 0.79; p < 0.0005, n = 15) and multiple stones (US: 3.8 +/- 0.8 vs fluid displacement: 3.7 +/- 0.8 ml; r = 0.85; p < 0.0001, n = 19).(ABSTRACT TRUNCATED AT 250 WORDS)