Abstract. The role of medical guidelines is becoming more and more important in the medical field. Within the Protocure project it has been shown that the quality of medical guidelines can be improved by formalisation. However formalisation turns out to be a very time-consuming task, resulting in a formal guideline that is much more complex than the original version and the relation with this original guideline is often unclear. This paper presents a case study where the relation between the informal medical guideline and its formal counterpart is investigated. This has been used to determine the gaps between the formal and informal guidelines and the cause of the size explosion of the formal guidelines.