Thirty-six patients were retrospectively followed an average of 25.1 months to evaluate the relative effectiveness of unilateral (16 patients) versus bilateral (20 patients) variable screw placement (VSP) instrumentation in isolate L4-L5 fusions. Demographic variables and preoperative diagnoses were similar between treatment groups. Outcome was assessed primarily through evaluation of plain roentgenograms and self-report questionnaires. Use of VSP instrumentation at the L4-L5 level with autogenous posterolateral grafting achieved a successful fusion rate of 97% with minimal complications. Fusion results with unilateral instrumentation were nearly identical to those of bilateral; in both cases, results were better than most historical controls for noninstrumented fusions in situ. Clinical outcome, as obtained through standardized measurement techniques of pain and function, demonstrated 69% excellent and good results. Clinical outcome was similar between treatment groups yet was not significantly related to the fusion status obtained at follow-up.