EMDIS is an open network between different bone marrow Donor Registries for patients awaiting a transplant and lacking compatible family donors. The network is composed of an identical system in all countries. The architecture makes clear definition between the user system (US) and the EMDIS core system (CS). E-mail approach is used as communication method. A logical model including the EMDIS CS and the US, as well as the list of messages exchanged between US/CS and CS/CS, has been defined. An actually functioning pilot system between three European countries could be used as a trial basis by other research projects.