The outcome in eleven patients in whom Grade-III and IV spondylolisthesis was treated non-operatively (Group I) was compared with that in twenty-one patients in whom the same degrees of spondylolisthesis were treated by posterior interlaminar fusion (Group II). At an average eighteen-year follow-up of the eleven patients in Group I, four (36 per cent) were asymptomatic, six (55 per cent) had mild symptoms, and only one had significant symptoms. Five (45 per cent) had one or more neurological findings, but none were incontinent. All of the patients in this group led an active life, and all had required only minor adjustments in their life-style. At an average twenty-four-year follow-up of the twenty-one patients in Group II, twelve (57 per cent) were asymptomatic, eight (38 per cent) had mild symptoms, and only one had significant symptoms. Nine (50 per cent) of the eighteen patients who had a physical examination had one or more neurological findings. Roentgenographically demonstrated failure of fusion did not adversely affect the results, and the patients remained asymptomatic despite the development of pseudarthrosis in one patient and bending of the fusion mass in three (14 per cent). In situ arthrodesis provides acceptable results for the patient who has Grade-III or IV spondylolisthesis and pain that interferes with life-style and that is unresponsive to non-operative treatment. In the skeletally immature patient, in situ fusion is recommended for disturbances of gait secondary to tight hamstrings and when the spondylolisthesis progresses.