Relative stretching of the cauda equina over the posterosuperior border of the sacrum can be found in all patients who have Grade-III or IV spondylolisthesis at the lumbosacral junction. We identified twelve patients, all less than eighteen years old, who had cauda equina syndrome after in situ arthrodesis for Grade-III or IV lumbosacral spondylolisthesis. In all twelve patients, posterolateral arthrodesis had been done bilaterally through a midline or paraspinal muscle-splitting approach. Nothing in the operative reports suggested that the cauda equina had been directly injured during any of the procedures. Five of the twelve patients eventually recovered completely. The remaining seven patients had a permanent residual neurological deficit, manifested by complete or partial inability to control the bowel and bladder. If dysfunction of the root of the sacral nerve is noted preoperatively in a patient who has lumbosacral spondylolisthesis, decompression of the cauda equina concomitant with the arthrodesis should be considered. An acute cauda equina syndrome that follows a seemingly uneventful in situ arthrodesis for spondylolisthesis is best treated by an immediate decompression that includes resection of the posterosuperior rim of the dome of the sacrum and the adjacent intervertebral disc. In addition, posterior insertion of instrumentation and reduction of the lumbosacral spondylolisthesis should be considered.