METHODS Prospective case series. OBJECTIVE To assess the results of instrumented posterolateral lumbar fusion, using recognized outcome assessment techniques, to evaluate the correlation between technical and clinical results and the effects of assessment techniques, and to compare the outcome with that of anterior lumbar fusion. BACKGROUND Assessments of lumbar spinal fusion results have frequently been published in forms that render direct comparison difficult and thus have not provided optimal assistance in the selection of a preferred method of treatment. METHODS One hundred and thirty-five patients with intractable back pain underwent instrumented posterolateral lumbar spinal fusion performed by a single surgeon. Review of results was undertaken by independent observers, using a recognized outcome assessment measure. RESULTS A solid bony fusion was obtained in 82% of patients. The method of outcome assessment profoundly affected the results; whereas 65% of patients rated themselves significantly improved by the procedure, only 19% achieved a good or excellent result on the Low Back Outcome Score. Patients undergoing a second procedure did not do well, and "salvage" surgery is not recommended. Compensation status was a significant prognostic factor. Psychological distress at review had a profound effect on the disability score and on patient satisfaction ratings. Overall, the results were inferior to those in a similar series treated by anterior lumbar fusion. CONCLUSIONS It is recommended that in future studies a recognized outcome score be used and that the analysis specifically include compensation status and psychological distress.