Myeloablative (MAC) as compared to reduced-intensity conditioning (RIC) is generally associated with lower relapse risk after allogeneic hematopoietic cell transplantation (HCT) for acute myeloid leukemia (AML) and myelodysplastic syndromes (MDS). However, disease specific risk factors in AML/MDS can further inform when MAC vs. RIC may yield differential outcomes. We analyzed HCT outcomes stratified by the disease risk index (DRI) in 4387 adults (age 40-65 years) to identify the impact of conditioning intensity. In the low/intermediate risk DRI cohort, RIC was associated with lower non-relapse mortality (NRM) (HR=0.74, 95% CI 0.62-0.88; p<0.001), but significantly higher relapse risk (HR=1.54, 95% CI 1.35-1.76; p<0.001) and thus inferior disease-free survival (DFS) (HR=1.19, 95% CI 1.07-1.33; p=0.001). In the high/very high risk DRI cohort, RIC resulted in marginally lower NRM (HR=0.83, 95% CI 0.68-1.00; p=0.051), and significantly higher relapse risk (HR=1.23, 95% CI 1.08-1.41; p=0.002) leading to similar DFS using either RIC or MAC. These data support MAC over RIC as the preferred conditioning intensity for AML/MDS with low/intermediate risk DRI, but similar benefit to RIC in high/very high risk DRI. Novel MAC regimens with less toxicity could benefit all, but more potent anti-neoplastic approaches are needed for the high/very high risk DRI group.