We reviewed radiographs and magnetic resonance images of 77 young athletes with spondylolysis and spondylolisthesis (more than 5% vertebral slip) (slip group). The results were compared with similar studies in 88 patients with spondylolysis only (nonslip group). Endplate lesions were found in all patients in the slip group and in 60 (68%) of those in the nonslip group. Slippage between the osseous and cartilaginous endplates was identified in the T1-weighted sagittal magnetic resonance images and categorized according to the type of slippage: total slip of L-5 or S-1, partial slip of L-5 or S-1, or a combination of these (mixed type). In a study of 31 patients whose slippages progressed, no slippage was associated with the early stage of a pars interarticularis defect. Most vertebral slippages developed or progressed in the cartilaginous or apophyseal stage of the lumbar skeletal age. Wedging of the L-5 vertebral body and rounding of the sacrum progressed as the slippage developed; these did not occur in the nonslip group. These results indicate that the advanced stage of a pars interarticularis defect in an immature spine is a risk factor for spondylolisthesis. The deformities of the lumbosacral spine are thought to be the secondary changes caused by vertebral slippage.