Résumé : The prognosis of steroid-refractory acute graft-versus-host disease (aGVHD) remains poor and better treatments are urgently needed. Multipotent mesenchymal stromal cell (MSC)-based therapy emerged as a promising approach but response rates were highly variable across studies. We conducted a multicenter prospective study assessing the efficacy of 1-2 infusion(s) of cryopreserved, third-party donor bone marrow-derived MSCs for treating grade II-IV steroid-refractory or -dependent aGVHD in a series of 33 patients. MSCs were produced centrally and distributed to 8 hospitals throughout Belgium to be infused in 2 consecutive cohorts of patients receiving 1-2 or 3-4 × 106 MSCs/kg per dose, respectively. All patients received MSCs as the first rescue therapy after corticosteroids, with the exception for one patient who received prior treatment with mycophenolate mofetil (that was still ongoing by the time of MSC therapy). In these conditions, MSC therapy resulted in at least a partial response in 13 patients (40.6%) at day 30 and in 15 patients (46%) within 90 days after first MSC infusion. The corresponding complete response rates were 21.6% (7 patients) and 30% (10 patients), respectively. Only 5 patients achieved a sustained complete response, lasting for at least 1 month. The 1-year overall survival was 18.2% (95% CI: 8.82-37.5%). Higher response and survival rates were observed among patients receiving 3-4 × 106 MSCs/kg for first infusion, as compared with patients receiving 1-2 × 106 MSCs/ kg. Response and survival with MSC therapy for SR/SD-aGVHD remains to be optimized.