Background: Extreme obesity (class III) is defined by the Centers for Disease Control as a body mass index (BMI) value !40. Recent studies suggest that obese patients have poor outcomes after thoracolumbar spinal fusions. The objective of this study was to analyze 30-day adverse events and patient-reported outcomes (PROs) for this population. Patients and Methods: A retrospective chart review of spinal fusion surgeries performed at a single institution from 2006 to 2016 was executed. All patients had a preoperative BMI !40. Patient characteristics, including age, sex, BMI, American Society of Anesthesiologists (ASA) score, Charlson Comorbidity Index (CCI), and others, were collected. Thirty-day adverse events (complications, readmissions, reoperations, and mortality) and PROs (Oswestry Disability Index [ODI] and visual analog scale [VAS]) were recorded. Results: Fifty-six patients were identified, including 30 men (54%). Mean age was 55.7 years (range, 31-74 years). Mean BMI was 44.2 (range, 40.0-54.7). Mean ASA was 2.7 (range, 2-3), and mean CCI was 1.1 (range, 0-6). Mean number of fused levels was 2.3 (range, 1-14). Mean length of stay was 4.4 6 2.1 days. Mean number of complications was 0.7 6 1.1, with 30.4% of patients having had at least 1 complication. The 30-day all-cause readmission rate was 5.4%, and 30-day reoperation rate was 3.6%. For 30 patients (54%) with 1-year PROs, mean preoperative ODI was 65.2 6 11.1, and mean preoperative VAS was 6.6 6 1.6. Mean ODI change was À19.9 6 20.1 (P , .001), and mean VAS change was À2.6 6 2.3 (P , .001). A total of 15 patients (50%) achieved the minimum clinically important difference in ODI (12.8), with a mean follow-up of 18.9 months. Conclusions: Patients with extreme obesity who undergo thoracolumbar fusion have acceptable 30-day adverse events and potentially can achieve significant improvement in pain and disability.