A double-blind, randomized trial was conducted to compare the efficacy of intradiskal injection of chymopapain (Chymodiactin) with injection of placebo in patients with a herniated lumbar disk. Patients were randomly assigned to either placebo or drug regimens and followed up for six months. The primary measure of performance was agreement by patient and surgeon that further intervention was not necessary. At any time that patient and investigator agreed that treatment had failed, the patient was classified as a failure and the treatment code was broken. Of 53 placebo-treated patients, 31 failed by this criterion. Of 55 drug-treated patients, 15 failed. Placebo-treated patients who failed were allowed to receive drug treatment, and 29 (91%) of 32 were treated successfully. This study demonstrates that chymopapain is more effective than placebo for treatment of patients with a herniated lumbar disk.