As huge amounts of knowledge are created rapidly, effective information access becomes an important issue. Especially for critical domains, such as medical and financial areas, efficient retrieval of concise and relevant information is highly desired. In this paper we propose a new user query based text summarization technique that makes use of Unified Medical Language System, an ontology knowledge source from National Library of Medicine. We compare our method with keyword-only approach, and our ontologybased method performs clearly better. Our method also shows potential to be used in other information retrieval areas.