Previous reports of posterolateral fusion for treatment of adolescent spondylolisthesis at L5-S1 suggest that persistent slippage is a significant problem in the postoperative period. Of 40 adolescent patients who underwent posterolateral fusion for L5-S1 spondylolisthesis, 23 demonstrated Grade 3 slip or worse. Two patients had progression of the slip postoperatively with subsequent spontaneous healing. All patients had complete incorporation of bone graft with resolution of neurologic deficits and pain. Improvement in results is attributed to the surgical technique and to postoperative casting. The displacement index, a new method to document relative L5-S1 position, is described.