The anatomy of the lumbar mamillo-accessory ligament (MAL) was studied by gross dissection in six cadavers. The MAL bridges the mamillary and accessory processes of each lumbar vertebra and encloses the medial branch of the dorsal ramus in an osseofibrous tunnel. The tunnel maintains the proximal course of the medial branch in a constant relationship to bone. This constancy allows for accurate percutaneous techniques to stimulate, anesthetize or destroy the medial branch. The MAL morphologically appears to represent remnants of transversospinal elements in the lumbar region, and is ossified in over 10% of lower lumbar vertebrae. Ossification may interfere with some percutaneous denervation techniques. The MAL may be a site of entrapment of the medial branch and may be a source of low-back pain.