Since 1980, many hospital information systems (HIS) came into being. These HIS are at the origin of the storage of huge medical data. Data mining is a domain which presents the most powerful tools for the extraction of knowledge and for structuring them. The first intelligent systems or assistance systems for medical diagnosis are the case-base reasoning systems. In these systems, the data mining method was integrated to improve them and to fill some gaps. We present an application of data mining which is based on case-base reasoning. The characteristic of this application is building knowledge base without integrating expert knowledge and we use this knowledge base to retrieve casefrom case base.