Mycophenolate mofetil (MMF) has shown synergistic effects in combination with cyclosporin A (CsA) in prevention of acute graft versus host disease (GvHD) after allogeneic blood stem cell transplantation (BSCT) in preclinical animal models. After having measured low plasma levels of the active metabolite mycophenolic acid (MPA) in recipients of allogeneic blood stem cell transplants after oral administration of MMF, we initiated a phase I/II study evaluating different dose levels of the intravenous (i.v.) formulation together with standard dose CsA.