A variety of techniques were used to study the morbid anatomy of 19 spines demonstrating degenerative spondylolisthesis in order to define the precise changes which produce this lesion and its radiographic appearance. An association between this affliction and vascular disease is suggested. The studies indicate that the listhesis is basically a persistent rotary deformity and not a simple forward displacement. Furthermore, there is no construction of the neural canal at the site of the listhesis. These findings suggest that the myelographic defect and symptomatology arise from traction applied to the nerve root by the medially displaced pedicle.