Abstract. HeCaSe2 is a multi-agent system that intends to help doctors to apply clinical guidelines to their patients in a semi-automatic fashion. HeCaSe2 agents need a lot of (scattered) information on health care organisations, as well as medical knowledge, in order to provide an efficient support to health care practitioners. Modelling all these data is certainly a hard task. The paper describes how the inclusion of an especially designed ontology allows different agents to coordinate their activities in the enactment of clinical guidelines.