The purpose of this study was to describe the relation of the lumbar plexus with the psoas major and with the superficial and deep landmarks close to it. Four cadavers were dissected and 22 computed tomography files of the lumbosacral region studied. Cadaver dissections demonstrated that the lumbar plexus, at the level of L5, is within the substance of the psoas major muscle rather than between this muscle and the quadratus lumborum. The femoral nerve lies between the lateral femoral cutaneous and obturator nerves. However, while the lateral femoral cutaneous nerve is in the same fascial plane as the femoral nerve, the obturator nerve can be found in the same plane as the two other nerves or in its own muscular fold. Radiological data provided the following measurements: the femoral nerve is at a depth of 9.01 +/- 2.43 cm; the psoas major medial border is at 2.73 +/- 0.64 cm from the median sagittal plane; and its lateral border is at 6.41 +/- 1.61 cm from the same plane. It is concluded that the lumbar plexus is within the psoas major, that the obturator nerve localization within the psoas major varies and that computed tomography data define precisely the relationship of the lumbar plexus with superficial and deep landmarks.