The authors report a case in which L5 radiculopathy developed acutely after surgery for placement of Harrington rod instrumentation for an L1 body fracture. Computed tomographic myelography demonstrated a large L4-L5 herniated disc that had not been present in preoperative studies. An emergency laminectomy was performed, and a large, free, subligamentous disc fragment was removed. The patient subsequently regained L5 sensorimotor function. The postoperative development of lumbar radiculopathy is an uncommon complication of Harrington rod instrumentation that may result from several biomechanical features of the instrumentation. These injuries may not be detected by intraoperative monitoring of somatosensory evoked potentials, and therefore, the postoperative neurological examination assumes a crucial role in the early diagnosis of these lesions. As our case demonstrates, these radicular deficits may be reversible if their cause is promptly recognized and treated.