In this paper the results of the standardization efforts and an evaluation study concerning electronic data interchange (EDI) performed by the 31 (Inter Institutional Information exchange)-project are described. In an earlier paper the results of a preliminary study were reported. EDI concerns communication between autonomous information systems and therefore needs standardization of the messages to be exchanged. In the 31-project ten different types of standard messages have been defined. The use of these messages for exchanging information between hospitals, GPs and pharmacies is described. It is concluded that relatively large time savings can be obtained when communicating electronically. It is also concluded that the electronic data interchange between hospitals and GPs can be performed without additional costs. The electronic communication of prescriptions between GPs and pharmacies does raise the costs of communication at the moment. The impact of EDI on the functioning of people in the organizations involved varies widely.