In a study designed to assess the diagnostic and prognostic usefulness of magnetic resonance (MR) imaging in patients to be treated with chymopapain, 39 patients with a clinicoradiographic picture of herniated lumbar disk were scanned. Twenty-one of these, who were actually treated, were rescanned 3 months following the intradiskal administration of chymopapain. In both pretreatment and posttreatment scans sagittal T1 and T2 weighted spinecho technique was used. Prior to therapy, disks that were later injected demonstrated narrowing of the intervertebral disk space (95% of cases); an extradural defect (86% of cases); diminished signal intensity on the T1 weighted images (50% of cases); and decreased signal intensity on the T2 weighted image when compared with normal controls (100% of cases). Additionally, the adjacent endplates of the injected disk showed increased signal intensity on the T1 weighted image in 32%; 18% of the cases displayed increased signal intensity on the T2 weighted image. Following chymopapain, 100% of injected disks demonstrated decrease in disk height; 36% of the extradural defects diminished in size and 9% increased; 91% of injected disks showed loss of signal intensity on the T1 weighted image; and 86% lost signal in the T2 weighted study with 14% remaining unchanged. Vertebral body endplates adjacent to the level of injection demonstrated a decreased signal intensity on the T1 weighted image in 32% of cases, and 40% demonstrated increased signal in the T2 weighted study. Even though these findings could not be correlated with clinical success or failure using the criteria in this study, six of the original 39 patients were diagnosed as having free disk fragments on the basis of their initial MR scans, a fact that does have management implications for the use of chymopapain in treatment of herniated disk disease.