The Etablissement français des Greffes (EfG) is a state agency dealing with Public Health issues related to organ, tissue and cell transplantation in France. The evaluation of organ retrieval and transplantation activities, one of its missions, is supported by a national information system (EfG-IS). The EfG-IS is moving towards a new n-tier architecture comprising a terminology server for end-stage diseases, organ failure, dialysis and transplantation (EfG-TS). Following a preliminary audit of the existing coding system and in order to facilitate data recording, to improve the quality of information, to assume compatibility with terminological existing standards and to allow semantic interoperability with other local, national or international registries, a specific work has been conducted on the thesauri to integrate within the EfG-TS. In this paper focusing on the server's content rather than the container, we report first the functional and cognitive requirements that resulted from the preliminary audit. We then describe the methodological approach used to build the terminological server on "sound ontological foundations". We performed the semantic analysis of existing medical terms to set up disease description frame-like structures. These diseases description frames consist of a limited set of nosological discriminating slots such as etiology, semiology, pathology, evolution and associated diseases. Each relevant medical term is thus associated to a concept defined and inserted within a hierarchy according to disease description frame resulting from the semantic analysis. Last, because this terminological server is shared by various transplant and dialysis centers to record patient data at different time point, contextualization of terms appeared as one of the functional requirements. We will also point out various contexts for medical terms and how they have been taken into account.