The cases of two patients with delayed paraplegia after segmental spinal instrumentation with sublaminar wiring are reported. Both patients had complex spinal deformities and had transient neural deficits after the first-stage procedure of anterior release and spine fusion. They had uneventful spinal-cord monitoring during the second-stage procedure of posterior instrumentation and fusion, and function of the lower extremities was present immediately after that operation. Paraplegia then ensued, and was recognized thirty hours later in one patient and six days later in the other. Considering our reproducible and reliable experience (no false-negative results) with spinal cord monitoring in 307 operations, we propose that the delayed onset of paraplegia resulted from a progression of ischemic and edema-producing events that had not developed sufficiently intraoperatively to be reflected by the monitoring. The paraplegia became evident only when the subarachnoid space was obstructed because of progressive postoperative neural edema. The presence of sublaminar implants in narrow, kyphotic segments of the spinal canal probably exacerbated the neural irritation by dural impingement, which was seen myelographically.