The appearance of the mesenteric vessels on computed tomographic scans was evaluated in 187 patients to determine the variations in the anatomic relationships between the mesenteric vessels in normal patients, in patients with malrotation, and in patients with abdominal masses. In 166 patients (88.8%), a portion of the cranial segment of the superior mesenteric vein (SMV) was anterior and to the right of the superior mesenteric artery (SMA), although other normal configurations also occurred. A portion of the cranial segment of the SMV was directly anterior to the SMA in 15 patients (8.0%) and directly to the right of the SMA in 16 patients (8.6%). Of eight patients with complete inversion of the SMA and SMV, one had normal midgut rotation, two had malrotation, and six had neoplasms that displaced the vessels (one of these six also had malrotation). Of eight patients with inversion of only a part of the mesenteric vessels, none had malrotation. Although mesenteric inversion should stimulate a careful evaluation for malrotation, it is not pathognomonic of this condition.