JAK inhibition by ruxolitinib is approved for treating myelofibrosis and also has shown efficacy in treating steroid-resistant acute and chronic graft-versus-host disease (GVHD). In 12 patients with myelofibrosis (median age, 63 years; range, 43 to 71 years) who were treated with ruxolitinib and underwent allogeneic stem cell transplantation (ASCT), ruxolitinib was continued (2 × 5 mg daily) until stable engraftment. No graft failure was observed, and leukocyte engraftment was achieved after a median of 12 days (range, 11 to 18 days). One patient developed fever of unknown origin after discontinuation of ruxolitinib; otherwise, no withdrawal syndrome was observed. Overall, only 1 patient each experienced acute GVHD grade I or II, resulting in an 8% incidence of acute GVHD grade II-IV at day +100, with no nonrelapse mortality. Complete chimerism was achieved in 11 patients after a median of 40 days, and molecular clearance of the underlying driver mutation was noted in 10 patients after a median of 32 days. Cytomegalovirus (CMV) reactivation occurred in 5 patients (41%), 1 of whom had CMV colitis as well, but all resolved after ganciclovir treatment. In 2 patients, ruxolitinib had to be discontinued on day 17 and day 18 after ASCT due to cytopenia after engraftment. Levels of inflammatory cytokines IL-8, IL-10, IL-6, TNFR2, INF-α, and INF-β were reduced after ruxolitinib treatment. After day +100, 4 patients developed acute GVHD (1 with grade I, 2 with grade II, and 1 with grade III) after tapering of cyclosporine, and all patients were alive at a median follow-up of 17 months (range, 12 to 18 months).