The purpose of this study, based on a consecutive series of 350 patients treated by chemonucleolysis in a 6-year period, is to analyze the causes of failure in 38 patients who underwent subsequent surgery. The most common cause of failure in this series was lateral recess stenosis, encountered in 16 of 38 patients. In contrast a sequestrated disc was found in only four patients. Failure of the enzyme to "digest" the protruded fragment was the cause of persisting sciatica in 11 other patients. No abnormality or recurrence at another level was found in the remaining patients. The success rate of surgery was 70%. The best results were obtained when a sequestrated disc or lateral stenosis was disclosed.