The postoperative radiographs of 48 patients who underwent posterolateral in situ fusion for spondylolysis and Grade 1 or less spondylolisthesis over an 11-year period showed healing of the pars defects in 90% of the cases. The patients who showed healing had an average preoperative duration of symptoms of 17 months (mean, 12 months). The five patients whose radiographs showed persistent defects postoperatively were found to have had preoperative symptoms for an average of 47 months (mean, 48 months). This significantly greater (P 0.05) duration of symptoms prior to stabilization was the only variable that proved to be statistically significant. Controversy continues to exist as to the etiology of spondylolysis; however, the ability to heal the defect with lumbar spinal stability certainly supports the theory that the defect is a result of fatigue stresses, which finally manifest as a fracture seen on radiographs as the spondylolytic defect. Posterior spinal stabilization in those patients with symptoms for a period of less than 2 years who have failed conservative management can be expected to heal the pars defects.