<jats:sec> <jats:title>Study Design.</jats:title> <jats:p>Retrospective review.</jats:p> </jats:sec> <jats:sec> <jats:title>Objective.</jats:title> <jats:p>To investigate associations of obesity with outcomes and costs of adult spinal deformity (ASD) surgery.</jats:p> </jats:sec> <jats:sec> <jats:title>Summary of Background Data.</jats:title> <jats:p>Increasing body mass index (BMI) is a risk factor for complications after nondeformity spine surgery, but its effect on ASD surgery is unknown.</jats:p> </jats:sec> <jats:sec> <jats:title>Methods.</jats:title> <jats:p>We reviewed records of 244 ASD patients who underwent spinal arthrodesis of ≥5 levels from 2010 to 2014 and categorized them by World Health Organization BMI groups: BMI < 30, nonobese (64%); BMI = 30–34.99, class-I obese (21%); and BMI ≥ 35, class-II/III obese (15%). We used multivariate logistical regression to determine odds of transfusion, inpatient complications, prolonged intensive care unit (ICU) stay (>2 days), prolonged total length of hospital stay (LOS) (>1 week), and high episode-of-care costs (>$80,000).</jats:p> </jats:sec> <jats:sec> <jats:title>Results.</jats:title> <jats:p>Preoperative characteristics were similar among groups, except sex, preoperative hemoglobin concentration, and performance/type of osteotomy (all, <jats:italic toggle="yes">P</jats:italic> = 0.01). On univariate analysis, the groups differed in rates of prolonged ICU stay (<jats:italic toggle="yes">P</jats:italic> < 0.001), prolonged total LOS (<jats:italic toggle="yes">P</jats:italic> = 0.016), and high episode-of-care costs (<jats:italic toggle="yes">P</jats:italic> = 0.013). Inpatient complication rates were similar among groups (<jats:italic toggle="yes">P</jats:italic> = 0.218). On multivariate analysis, compared with nonobese patients, class I obese patients had greater odds of prolonged ICU stay (odds ratio [OR] = 2.24, 95% confidence interval [CI]: 1.06, 4.71). Class II/III obese patients also had greater odds of prolonged total LOS (OR = 2.21, 95% CI: 1.03, 4.71), and high episode-of-care costs (OR = 2.91, 95% CI: 1.31, 6.50).</jats:p> </jats:sec> <jats:sec> <jats:title>Conclusion.</jats:title> <jats:p>In ASD surgery, BMI ≥35 is associated with significantly worse perioperative outcomes and higher costs compared with those of nonobese patients.</jats:p> <jats:p> <jats:bold>Level of Evidence:</jats:bold> 3</jats:p> </jats:sec>