We demonstrate the BioNav system, a novel search interface for biomedical databases, such as PubMed. BioNav enables users to navigate large number of query results by categorizing them using MeSH; a comprehensive concept hierarchy used by PubMed. Once the query results are organized into a navigation tree, BioNav reveals only a small subset of the concept nodes at each step, selected such that the expected user navigation cost is minimized. In contrast, previous works expand the hierarchy in a predefined static manner, without navigation cost modeling. BioNav is available at http://db.cse.buffalo.edu/bionav. Categories and Subject Descriptors H.3.3 [Information Storage and Retrieval]: Information Search and Retrieval ? search process, selection process and information storage and filtering. H.3.6 [Information Storage and Retrieval]: Digital Libraries ? Dissemination, User Issues. J.3 [Computer Applications]: Life and Medical Sciences ?Medical Information Systems. General Terms Algorit...