STUDY DESIGN: Retrospective cohort study using Thomson Reuter's MarketScan database. OBJECTIVE: To evaluate the extent to which Medicaid versus commercial insurance status affects outcomes after lumbar stenosis surgery. SUMMARY OF BACKGROUND DATA: The Affordable Care Act aims to expand health insurance and to help narrow existing health care disparities. Medicaid patients have previously been noted to be at an increased risk for impaired access to health care. Conversely, those with commercial insurance may be subject to overtreatment. We examine the surgical treatment of low back pain as an example that has raised significant public health concerns. METHODS: A total of 28,462 patients, ages 18 and older, were identified who had undergone laminectomy or fusion for spinal stenosis between 2000 and 2009. Patients were characterized by baseline demographic information, comorbidity burden, and type of insurance (Medicaid vs. commercial insurance). Multivariate analysis was performed comparing the relative effect of insurance status on reoperation rates, timing and type of reoperations, postoperative complications, and total postoperative health resource use. RESULTS: Medicaid patients had similar reoperation rates to commercially insured patients at 1 year (4.60% vs. 5.42%, P = .38); but had significantly lower reoperation rates at 2 (7.22% vs. 10.30%; adjusted odds ratio [aOR] = 0.661; 95% confidence interval [CI], 0.533-0.820; P = .0002) and more than 2 years (13.92% vs. 16.89%; aOR = 0.722; 95% CI, 0.612-0.851; P <.0001). Medicaid patients were particularly less likely to undergo fusion as a reoperation (aOR = 0.478; 95% CI, 0.377-0.606; P < 0001). Medicaid patients had greater health care resource utilization as measured by hospital days, outpatient services and medications prescribed; however, commercially insured patients had significantly higher overall health utilization costs at 1 and 2 years. CONCLUSION: There are insurance disparities that affect important surgical outcomes after initial surgery for spinal stenosis. Efforts for national health care reform should include explicit efforts to identify such system factors that will reduce current inequities in care. LEVEL OF EVIDENCE: 2.