Gallstones from 63 patients were evaluated by in vitro 1.5-T MR imaging, with T1- and T2-weighted images, and in 14 cases, a fat-suppression sequence (short-T1 inversion recovery imaging). Subsequent chemical analysis was performed on 43 gallstones. In vitro proton MR spectroscopy was performed on 14 stones. On T1-weighted MR images, foci of increased signal were seen in 46 of 63 stones (faint in 17, moderate in nine, and bright in 20). T2-weighted images showed areas of increased signal in 18 of 63 stones (faint in 15, moderate in three). T1-weighted MR imaging patterns were homogeneously dark (17), homogeneously bright (two), homogeneously faint (three), rimmed (dark rim and bright center, 32), and laminated (nine). Short-T1 inversion recovery imaging suppressed the foci of increased signal in 13 of 14 cases. Despite imaging characteristics suggestive of high lipid content, spectroscopy revealed only a single peak corresponding to a large water-proton signal. The T1 relaxation times of the water were shortened, ranging from 0.006 to 0.92 sec, explaining the increased signal seen on MR images of the gallstones. MR imaging characteristics (signal intensity, relative signal area, or imaging patterns) did not correlate with chemical composition. We hypothesize that different structural relationships must exist within gallstones of similar chemical content that alter the water bonding and hence the MR imaging characteristics.