Clinical trials are essential to verify the effectiveness of medical treatments and procedures. In order to be considered valid, a clinical trial needs to involve a large number of patients with specific conditions. Selecting enough patient with matching conditions is hard, and many clinical trials fail because of the lack of participants. In the case of cancer, the decision of putting a patient on a trial is taken during multi-disciplinary meetings, where different aspects of each case are collectively reviewed in few minutes. In this presentation, I will discuss an approach that uses a domain-specific ontology represented in OWL, together with SWRL rules, to represent patient data and selection criteria for clinical trials. The final objective is the automatic identification of clinical trials matching patients' conditions.