Gallbladder contraction in response to a liquid fatty meal was studied in a hundred consecutive adult Thai subjects by using an ultrasound machine. None of them had clinical evidence of hepato-biliary and gastric diseases. The gallbladder sizes were detected at the fasting stage and after taking a liquid fatty meal (250 ml of milk) at 30, 60, 90 and 120 minutes respectively. The gallbladder size was measured by two methods, the surface area (SA) along its longitudinal plane and gallbladder volume (VOL). The percentage of maximal reduction of gallbladder size in each individual was considered as the gallbladder contraction capacity. The average contraction capacity in this group was 57.88% +/- 12.38% by SA or 74.47% +/- 9.96% by VOL. The variation of gallbladder contraction capacity in this population was found to have a normal curve distribution. The maximal contractions were found mostly (69%) at 60 minutes and 93% of subjects had maximal contractions within 90 minutes. There was no difference in gallbladder contraction capacity between males and females and also among different age groups. The tenth percentile of gallbladder contraction capacity was 41.12% by SA or 61.82% by VOL. We propose that these should be considered as the lower limit of normal gallbladder contraction capacity after taking 250 ml of liquid fatty meal. This test was simple, safe and easily measured. This may provide useful basic information for further functional study of gallbladder diseases.