Computer tomographically guided discography was used as a reproducible and reliable method of determining the internal architecture and integrity of intervertebral discs before spinal fusion operation. One hundred disc levels were studied in 60 prospective patients with 54 (54%) of levels proving abnormal. Twenty-nine levels (29% of total levels) demonstrated complete anular tears, and 25 (25% of total) showed incomplete radial tears of the anulus, often multiple, indicative of degenerative change. In each of these cases, one level higher had to be studied and was subsequently found to be normal and would, therefore, support a fusion.