Minor defects of the sacrum are common and often dismissed as normal. The radiographs of 100 patients with urinary incontinence revealed sacral abnormalities in 43. Sacral evoked responses were measured and compared independently with the radiological findings. Patients who had defective closure of the dorsal neural arches of S1 or S2 all had abnormal nerve studies. Incomplete development of the bones of the dorsal neural arches of the upper sacrum may be a marker of incomplete neurogenesis of the sacral nerves. The sacral neurological deficit is subtle and difficult to demonstrate, but it is real and may be important in the pathogenesis of incontinence.