In a series of 34 patients with herniated lumbar discs, treated by intradiscal injection of highly purified collagenase, the post-treatment course has been followed-up clinically and by repeated computed tomographies (CT). Good or excellent results have been achieved in 17 patients. An only slight improvement of pain was noted in 2 patients. Fifteen patients had to be operated on due to not improved or worsened clinical symptoms. The most striking result of our CT follow-up was a tendency of the disc herniation to increase initially after collagenase injection. About two thirds of the patients had such an increase at the one week after injection control. After 6 weeks this rate had decreased to only about one quarter, but in the meantime 13 patients had to be operated. Only after 6 months most hernias of the up till then not operated patients were smaller and none were larger than before treatment. There was also a transient density decrease of the treated disc, most pronounced one week after collagenase injection. At controls 6 months later density had reached again pre-treatment levels. It is likely that the volume increase tendency of the disc material after collagenase injection is responsible for a worsening of the clinical symptoms, which not seldomly occurs during the initial post-treatment period, and in some patients makes an operation necessary.