We investigated sirolimus and mycophenolate mofetil (MMF) as GVHD prophylaxis in patients with advanced hematological malignancies receiving myeloablative hematopoietic cell transplantation (HCT) from HLA-identical sibling donors. On the basis of pre-study stopping rules, the trial was closed to accrual after enrollment of 11 adult patients. In all, 7 of the 11 patients received BU-containing preparative regimens. Sirolimus was discontinued in three patients because of the toxicity-related events of severe sinusoidal obstructive syndrome, portal vein thrombosis, altered mental status and in one patient because of the risk of poor wound healing. In all, 6 of the 11 patients developed grade II-IV acute GVHD (AGVHD) a median of 15.5 days post HCT. Two of three patients with grade IV AGVHD had sirolimus discontinued by 9 days post HCT. All patients responded to AGVHD therapy without GVHD-related deaths. There were two non-relapse- and two relapse-related deaths. At a median follow-up of 38 months (2-47 months), 7 of 11 patients were alive without disease. MMF and sirolimus GVHD prophylaxis did not reduce the risk of AGVHD, however, there were no GVHD-related deaths. The severe toxicities in the patients receiving the BU-containing preparative regimens limited the continued use of sirolimus and MMF for the prevention of AGVHD.