The Differential diagnosis (ddx) is a systematic method to identify unknowns. This method, essentially a process of elimination, is used by taxonomists to identify living organisms and by physicians to diagnose the specific disease in a patient. One important point is the enormous amount of knowledge that doctors are required to possess, in order to be able to make a correct ddx. The main problem is that the number of diseases which exist worldwide is enormous, and it is beyond the scope of the human brain to remember them all. For this reason, in this paper we present MEDBOLI, an ontology-driven medical diagnosis system which applies the use of ontologies combined with logical inference and computation of probabilities to establish the probability of the diagnosis.