Antifungal prophylaxis with azoles is considered standard in allogeneic hematopoietic SCT (allo-HSCT). Although sirolimus is being used increasingly for the prevention of GVHD, it is a substrate of CYP3A4, which is inhibited by voriconazole, and concurrent administration can lead to significantly increased exposure to sirolimus. We identified 67 patients with hematologic malignancies who underwent allo-HSCT with sirolimus, tacrolimus and low-dose MTX and received concomitant voriconazole prophylaxis from April 2008 to June 2011. All patients underwent a non-myeloablative or reduced-intensity conditioned allo-HSCT. Patients received sirolimus and voriconazole concurrently for a median of 113 days. The median daily dose reduction of sirolimus at the start of coadministration was 90%. The median serum sirolimus trough levels before and at steady state of coadministration were 5.8 ng/mL (range: 0-47.6) and 6.1 ng/mL (range: 1-14.2) (P=0.45), respectively. One patient with an average sirolimus level of 6 ng/mL developed sirolimus-related thrombotic microangiopathy that resolved after sirolimus discontinuation. No sinusoidal obstructive syndrome was reported. Seventeen patients (25%) prematurely discontinued voriconazole because of the adverse events. Only two patients (3%) presented with possible invasive fungal infections at day 100. We demonstrate that sirolimus and voriconazole coadministration with an empiric 90% sirolimus dose reduction and close monitoring of sirolimus trough levels is safe and well tolerated.