A prospective study of a consecutive series of lateral mass fusion operations was undertaken between 1972 and 1982 in 144 patients, of whom 129 were available for review. The main indications for operation were persistent back pain associated with spondylolisthesis, localized degenerative disease of the lumbar spine and previous laminectomy and disc excision. The minimum follow-up was 2 years. Although 83% of patients were improved overall, the results in spondylolisthesis and degenerative disease without previous surgery were much better than in those who had previously had disc excision. A pseudarthrosis was found in 13% of patients and was significantly more likely (P less than 0.05) when fusion at more than one level was attempted. There were no serious long-term complications.