Histologic findings of 4 cases of spondylolysis and spondylolisthesis in childhood and their pathogenetic interpretation. The microscopic changes in the lateral gap of the vertebral arch in children are explained as due to stress in the growing intervertebral bones, as fatigue fractures in the widest sense, with a tendency to pathologic union and fibro-cartilagineous pseudarthrosis. During pathogenesis we distinguish between the "primary lysis", a stage of repair followed by defective healing and the resulting spondylolysis.