Computerised tomography (CT) was performed in 30 patients with herniated lumbar discs; this was done both before chemonucleolysis and three months after. In 20 of the 28 patients who were CT positive, the compression produced by the herniated disc was eliminated or reduced. Twenty-three of the 28 patients developed diffuse bulging of the annulus. There was good correlation between the clinical results at three months and the alteration in compression as shown by the CT scan. No evidence of alteration in the bony relationship was seen in the scan and none of the patients developed epidural fibrosis. Chemonucleolysis has thus been shown to be an effective treatment of herniated lumbar discs, but it is definitely not indicated in cases where compression of the nerve root or dural sac is due to a bulging annulus.