Sirolimus (SIR)/tacrolimus (TAC) is an alternative to methotrexate (MTX)/TAC. However, rational selection among these GvHD prophylaxis approaches to optimize survival of individual patients is not possible based on current evidence. We compared SIR/TAC (n=293) to MTX/TAC (n=414). The primary objective was to identify unique predictors of overall survival (OS). Secondary objective was to compare acute and chronic GvHD, relapse, non-relapse mortality, thrombotic microangiopathy (TMA), hepatic veno-occlusive disease (VOD/SOS), and acute kidney injury. Day 100 grades II-IV acute GvHD was significantly reduced in SIR/TAC vs MTX/TAC group (63 vs 73%, P=0.02). An interaction between GvHD prophylaxis groups and comorbidity index (hematopoietic cell transplantation (HCT)-CI) significantly impacted OS. Patients with HCT-CI' 3/44 had significantly worse OS with MTX/TAC (HR 1.86, 95% CI 1.14-3.04, P=0.01) while no such effect was seen for SIR/TAC (HR 0.78, 95% CI 0.48-1.26, P=0.31). Other end points did not significantly differ between groups except TMA and VOD/SOS were increased in the SIR/TAC group, but excess death from these complications was not observed. We conclude, GvHD prophylaxis approach of SIR/TAC is associated with reduced grades II-IV acute GvHD, comparable toxicity profile to MTX/TAC, and improved OS among patients with HCT-CI' 3/44.