© 2016 Elsevier Inc. Objectives Body mass index (BMI) commonly is used in obesity classification as a surrogate measure, and obesity is associated with a cluster of risk factors for cardiovascular disease. The aim of this study was to investigate the impact of BMI on short-term outcomes after cardiac surgery. Design A retrospective cohort study. Setting University teaching hospital, 2 centers. Participants The study comprised 4,740 patients who underwent cardiac surgery at 2 hospitals—from July 1, 2001, to June 30, 2013, in 1 hospital and from September 1, 2003, to August 31, 2014, in a second hospital. Interventions No changes to standard practice were required. Measurements and Main Results Patients were assigned into 6 BMI groups as follows: underweight (BMI<18.5 kg/m2), normal weight (≥18.5 to<25 kg/m2), overweight (≥25 to<30 kg/m2), class I obese (≥30 to<35 kg/m2), class II obese (≥35 to<40 kg/m2), and class III obese (BMI≥40 kg/m2). Short-term major postoperative complications (postoperative stroke, cardiac arrest, new atrial fibrillation/flutter, permanent rhythm device insertion, deep sternal infection, sepsis, prolonged ventilation, pneumonia, renal dialysis, renal failure, intensive care unit readmission, total intensive care unit hours, and readmission in 30 days, and mortalities (in-hospital mortality, 30-day mortality, surgical mortality) were compared among various BMI groups after cardiac surgery. Age, sex, surgery type, family history of coronary artery disease, diabetes, hypertension, heart failure, and lipid-lowering medication were the risk factors for early outcomes. Multiple logistic regression analysis indicated that the underweight and class III obese BMI groups demonstrated significant, adverse differences in some short-term outcomes, including deep sternal infection, prolonged ventilation, new atrial fibrillation/flutter, and renal failure. However, being in the overweight or class I obese group demonstrated a positive effect on discharge and surgical mortality. Conclusions The results of this study demonstrated that extreme obesity and underweight were significantly associated with early major adverse clinical outcomes. However, there was an “obese paradox” in short-term mortality after cardiac surgery.