Abstract 4536 Objective To evaluate the safety profile and efficacy of umbilical cord-derived mesenchymal stem cell infusion in patients with steroid-resistant, severe, acute graft-versus-host disease (aGVHD). Methods A total of 19 patients with steroid-resistant severe aGVHD received mesenchymal stem cell infusion treatment. We analyzed the treatment response, transplantation-related mortality, events associated with infusion and relapse rate. Results Two patients with grade II, 5 patients with grade III and 12 patients with grade ‡W aGVHD received a total of 58 infusions of mesenchymal stem cell. The mean total dose of mesenchymal stem cell was 2.13×10 6 (range 0.6–7.2×10 6 ) cells per kg bodyweight. 7 patients received one infusion, 2 patients received two infusions, and 10 patients received three or more infusions. 11 patients had a complete response and 4 had a partial response and 4 had no response. No patients had side-effects during or immediately after infusions of mesenchymal stem cell and no ectopic tissue was detected to date. 11 patients survived and 8 died, 4 for aGVHD, 1 for infection and 2 for aGVHD with concomitant infection and 1 for underlying leukemia relapse. The cell viability of freshly prepared mesenchymal stem cell is 93% (92%-95%) by trypan blue staining. The cell viability of controlled-rate freezed and thawed cells mesenchymal stem cell is 72% (70%-74%). Conclusion Infusion of umbilical cord-derived mesenchymal stem cell expanded in vitro is an effective therapy for patients with steroid-resistant, severe aGVHD without negative impact on relapse. Freshly prepared mesenchymal stem cells are superior to freezed and thawed cells in terms of cell viability. Disclosures: No relevant conflicts of interest to declare.