We reviewed 40 spinal fusions done prior to Risser stage I for idiopathic and paralytic scoliosis to evaluate postoperative curve progression. The 39 patients who had posterior fusion alone had progressive angulation and rotation of the spine over the postoperative follow-up period. The more immature the patient, the greater the resultant progression. This progression is an inevitable consequence of continued anterior spinal growth in the presence of a posterior fusion, and occurs without pseudarthrosis or hardware failure. Younger patients may require anterior and posterior fusion to achieve stable correction.