Traditional CBR approaches imply centralized storage of the case base and, most of them, the retrieval of similar cases by an exhaustive comparison of the case to be solved with the whole set of cases. In this work we propose a novel approach for storage of the case base in a decentralized Peer-toPeer environment using the notion of Unspecified Ontology. In our approach the cases are stored in a number of network nodes that is comparable with the number of cases. We also develop an approximated algorithm for efficient retrieval of most-similar cases. The experiments show that the approximated algorithm successfully retrieves the most-similar cases while reducing the number of cases to be compared.