A treebank is a corpus of tagged and bracketed sentences capturing the linguistic properties of a (sub)language in an empirical way. The CASSANDRA treebank is developed as a sideline to the GALEN-IN-USE project in which it serves to make the relationships between natural language phenomena and semantic representations of medical expressions explicit, and to assist in the quality assurance of the modelling centres. The end result is a multilingual linguistic knowledge repository from which lexicons and grammars of various types can be derived in an automatic way.