Successful design of information systems in health care starts with a thorough understanding of the practices in which the systems are to function. In this paper, we discuss the nature of 'medical information' from a sociological perspective. We focus on the (im)possibilities of the utilization of primary health care data for secondary purposes such as research and administration. In much of the literature on EPRs, this secondary utilization is only seen to depend on the question whether the IT connections are in place. It is then simply a matter of selecting which information to transport and to where. In this article, we argue that this view of medical information is mistaken. Information should be conceptualized as always entangled with the context of its production. The disentangling of information from its production context is possible, but that entails work. We propose the following 'law of medical information': the further information has to be able to circulate (i.e. the more diverse contexts it has to be usable in), the more work is required to disentangle the information from the context of its production. The question that then becomes pertinent is; who has to do this work, and who reaps the benefits?