Gallbladder contractility can be quantified radiologically, but it is not known whether the degree of contraction exhibited by a person's gallbladder varies from day to day. Thirty healthy volunteers were studied with sonography on three separate occasions to determine the variation of individual gallbladder contractility. Using the ellipsoid method, we measured gallbladder volume after an overnight fast (fasting gallbladder volume) and between 45 and 60 min after a standard fatty meal (residual gallbladder volume). Percentage gallbladder contraction was calculated by dividing the difference between the fasting and residual gallbladder volumes by the fasting gallbladder volume and multiplying by 100. The 90 studies in 30 subjects exhibited a wide range of values: fasting gallbladder volume from 1.9 to 45.5 ml, residual gallbladder volume from 0.1 to 21.0 ml, and percentage gallbladder contraction from -10% to 99%. Within each subject, fasting gallbladder volume measurements varied from 1.5 to 26.2 ml (mean +/- two standard deviations, 10.3 +/- 5.1 ml) and residual gallbladder volume from 0.3 to 15.4 ml (5.1 +/- 3.8 ml). Percentage gallbladder contraction varied from 6% to 87% (28% +/- 18%). In 60% of the subjects, percentage gallbladder contraction values varied by more than 20%, and in 20% of the subjects it varied by more than 40%. These data show that a wide variation exists within a normal person in the degree of gallbladder contraction exhibited from one day to another, and a single test for gallbladder contraction can be misleading.