In recent years, medical professionals are witnessing an explosive growth in data collected by various organizations and institutions. At the same time, the ongoing developments of networking technologies provide doctor with the capability to access these data across the boundaries of interconnected computers. In this paper we present a medical data warehousing methodology that aims to use data semantics to regroup and merge patients' medical data from different health information systems, which may be autonomous and heterogeneous. The proposed solution takes into account European laws concerning the security and anonymity of personal data.