Changes in the intervertebral disc after chemonucleolysis in 26 patients were observed by magnetic resonance imaging for 1 year, and from the results a mechanism for chemonucleolysis was considered. A reduction in size of the herniation was not recognized 2 weeks after treatment, when chymopapain had caused necrosis of the disc, but began after 4 weeks, when regeneration of the disc had occurred. The herniation was further reduced after 3 months, and the disc had become fibrocartilaginous in a cicatricial process. Marked decrease in disc signal was observed after 2 weeks, and never recovered. From these facts, it is considered that chymopapain might never cause primary diminution of disc herniation, and any reduction of herniation that occurs is due to the effects of a secondary cicatricial contraction.