Regions of sensory impairment of 71 patients in 86 lumbar spinal nerve blocks (L4 19 cases, L5 41 cases, S1 26 cases) were determined by the writing brush method. Distinctive regions included the medial side of the lower leg, the side of the first dorsal digit, and the side of the fifth digit respectively of the L4 (88%), L5 (82%) and S1 (83%) spinal nerves without 100% matching because of displacement of one spinal segment by the furcal nerve. A band-like impaired zone from the proximal to the periphery appeared in 42%, 44%, and 92%, respectively, of the L4, L5, and S1 blocks. The marked difference between L4, L5, and S1 may be caused by the diversity of the dorsal rami of the spinal nerves and double control by the lateral cutaneous nerve of the thigh.