The objective of this study was to determine the level of the aortic bifurcation in relation to the lumbar spine by MRI and the effect of lumbosacral anomalies on the aortic bifurcation. A prospective study of 441 patients was performed. Sagittal MR images of the entire spine were obtained along with the standard protocol for imaging of the lumbar spine. The vertebrae were counted caudally from C2 instead of cranially from the presumed L5 vertebra. The aortic bifurcation in relation to the lumbar vertebrae was determined. The aorta bifurcated at the L4 vertebral body in 67% of cases. In patients with sacralization of L5 the aortic bifurcation was at the L3 vertebral body in 59%. In those patients with lumbarization of S1 the aorta bifurcated at the level of the L4 vertebral body in 40% and at the L4/5 disc space in 33%. There was no demographic variation of the aortic bifurcation in relation to age or sex. The aorta bifurcated at L4 in two-thirds of cases and was variably located in the remaining third. The stability of this as a landmark is disturbed by the significant high incidence of lumbosacral transitional segments.