OBJECTIVE To describe the MR manifestations and temporal course of wallerian degeneration that occurs above and below a spinal cord injury, and to compare the MR findings with postmortem histopathology. METHODS Twenty-four postmortem spinal cords from patients with cervical (n = 14), thoracic (n = 6), and lumbar (n = 4) cord injuries were studied with axial T1- and T2-weighted spin-echo MR imaging. Injury-to-death intervals varied from 8 days to 23 years. The images were examined for alteration of signal above and below the injury site. Histologic studies of these cords with axon, myelin, and connective tissue stains were performed at levels equivalent to the MR sections. Immunohistochemical analysis using antibodies to glial fibrillary acetic protein was also performed on 19 cords. Pathologic-imaging comparisons were made. RESULTS MR images showed increased signal intensity in the dorsal columns above the injury level and in the lateral corticospinal tracts below the injury level in all cases in which cord injury had occurred 7 or more weeks before death. In early postinjury survival times (8 days and 12 days) MR findings were normal; histologically there was early wallerian degeneration in only the dorsal columns at 8 days and in both the lateral and dorsal columns at 12 days. MR showed wallerian degeneration in all cases examined at 7 weeks after injury and thereafter. CONCLUSIONS Wallerian degeneration was demonstrated by histology and MR in all specimens in which the injury-to-death interval was greater than 7 weeks. Recognition of wallerian degeneration on MR allows complete analysis of the injury, explains abnormal MR signals at sites remote from the epicenter of the injury, and may be useful in the future in the timing and planning of therapeutic interventions.