The defect in the pars interarticularis in spondylolysis and spondylolisthesis is most often the result of repeated trauma, stress, and factors other than acute fracture. These fatigue fractures develop early in life, may have a strong hereditary basis, and most often represent incidental roentgenographic findings. Attention should be given to the youngster or adolescent with low-back pain and paraspinal muscle spasm. If these patients are followed closely, the incidence of pars interarticularis defect is higher than appreciated. The lesion in some of these individuals may progress to significant vertebral slipping. If the developing defect is recognized early, treatment can be quite satisfactory.