There has been ongoing debate on the association between obesity and outcomes in acute myeloid leukemia (AML). Currently there are few studies that have stratified outcomes by class I obesity, class II obesity, and class III obesity; and a more nuanced understanding is becoming increasingly important with the rising prevalence of obesity. We examined the association between body mass index (BMI) and outcomes in previously untreated AML in younger patients (age ≤60) enrolled in SWOG S1203 (n=729). Class III obesity was associated with an increased rate of early death (p=0.004) and worse overall survival (OS) in multivariate analysis (hazard ratio (HR) 2.48, 95% confidence interval (CI) 1.62-3.80 versus normal weight). Class III obesity was also associated with worse OS after allogeneic hematopoietic cell transplant (HR 2.37, 95% CI 1.24-4.54 versus normal weight). These findings highlight the unique risk of class III obesity in AML, and the importance of further investigation to better characterize this patient population.