Sinusoidal obstruction syndrome (SOS) is a serious complication of hematopoietic stem cell transplantation (HSCT). Sirolimus plus tacrolimus is an accepted regimen for graft-versus-host disease (GVHD) prophylaxis, with both agents implicated as risk factors for SOS. We analyzed 260 consecutive patients who underwent allogeneic HSCT following myeloablative conditioning using total body irradiation (TBI)-based (n = 151) or chemotherapy only (n =
109) regimens, with sirolimus plus tacrolimus for GVHD prophylaxis. SOS occurred in 28 patients at a median of 22 (range, 12-58) days. Mean sirolimus trough levels were higher between days 11 and 20 following transplant in patients who developed SOS (10.3 vs. 8.5 ng/ml, P = 0.008), with no significant difference in mean trough levels between days 0 and 10 (P = 0.67) and days 21-30 (P = 0.37). No differences in mean tacrolimus trough levels during the same time intervals were observed between those developing SOS and others. On multivariable analysis, a mean sirolimus trough level ≥ 9 ng/ml between days 11 and 20 increased the risk of SOS (hazard ratio 3.68, 95% CI: 1.57-8.67, P = 0.003), together with a longer time from diagnosis to transplant (P = 0.004) and use of TBI (P = 0.006). Our results suggest that mean trough sirolimus levels ≥ 9 ng/mL between days 11 and 20 post transplant may increase the risk of SOS and should be avoided.