Longitudinal insertion of a lumbar puncture needle bevel is less likely to cause post-lumbar puncture headache than is transverse insertion. The reason for this has not been entirely clear. We investigated the direction of dural fibers in posterior L3-4 dura mater obtained from three autopsies and tallied it in three orthogonal planes. Under low-power light microscopy the tissue appeared to be composed of lamellae branching irregularly and directed concentrically to the spinal cord. Electron micrographs (x5000) revealed that the lamellae consisted mainly of bundles of collagen fibers that pursued wavy courses in various directions. The lamellae also contained branching elastic fibers, many of which were directed longitudinally. This arrangement of fibers probably minimizes the tendency of a dural puncture hole to gape under tension if the needle bevel is directed longitudinally.