The lumbo-sacral ligament (LSL) was studied in 42 specimens. It extends from the L5 vertebra to the ala of the sacrum and forms, with the structures to which it is attached, an osteofibrotic tunnel as an extension of the intervertebral foramen. The 5th lumbar nerve root passes through the tunnel over the ala of the sacrum and behind the LSL. A branch of the 4th lumbar nerve root passes in front of the LSL to join the 5th below the ligament to form the lumbo-sacral trunk. The sympathetic ramus communicans to the L5 root always penetrates the LSL at its superior border and reaches the nerve inside the tunnel. Branches of the ilio-lumbar vessels accompany the L5 root. Large osteophytes on the inferior border of L5 and tightness of the LSL were often found to cause entrapment and compression of the L5 nerve root against the ala of the sacrum.