In this work, we present a new fusion method based on fuzzy set theory. This method consists of combining several data and knowledge bases of diagnostic systems. It is characterized by a hybrid fusion, which combines base fusion of data and knowledge of the Case-Based Reasoning diagnostic systems. The fusion method relies on distortion measure of various diagnostic systems (of case and knowledge bases). This distortion measure is integrated into the diagnostic system in order to improve its performance. It is defined by confidence degrees associated to each parameter that composes the case and knowledge bases of diagnostic systems. The confidence degrees are then integrated into the diagnostic system procedure.