Fifty consecutive patients underwent standardized surgical treatment for isthmic lumbosacral spondylolisthesis. Twenty-two (44%) had mechanical symptoms only and were treated with in situ fusion. Twenty-eight (56%) had back and radicular symptoms and underwent decompression and fusion. Follow-up averaged 40.4 months. Satisfactory results were achieved in 30 (60%). Patients under 30 and over 50 appeared to do better. Success rate was not related to degree of slippage. Success rate in compensation cases was 39%, versus 83% in non-compensation cases (P< 0.001); males, 53%, versus females, 78%(0.05< P< 0.1); back pain only, 73%, versus radiculopathy, 50%(0.05< P< 0.1); smokers, 48%, versus nonsmokers, 74%(0.05< P< 0.1). Pseudarthrosis rate was 12%, and this correlated with failure (P< 0.002). Thus, a trend towards an unsatisfactory outcome was seen in males, middle-aged individuals, those with a smoking habit, and patients with radicular symptoms. A compensable work situation and pseudoarthrosis had a profoundly negative influence on outcome.