The definition of common system semantics is an explicit and generally accepted precondition for comparability and exchangeability of data from different systems. We have looked back on 15 years of experience with a data model that was developed in a co-operative effort by experts from various hospital cancer registries as the foundation of a new tumour documentation system (GTDS). The data model is based on the definition of a common basic data set for hospital cancer registries which is agreed by the German Association of Comprehensive Cancer Centres (ADT). This paper presents an “entity relationship” view of this model. Since data exchange among registries and with hospital or practice information systems is becoming increasingly important we describe our method to import data from such systems. We discuss the requirements that systems have to have for a most effective way of exchanging data with a hospital cancer registry. The most important feature is the possibility to associate disease phenomena and therapies with each other and with an entity that represents the tumour across encounters. The reference model we present respectively the requirements we propose for other communicating systems might also fit for other chronic diseases.