The Unified Medical Language System (UMLS) is a rich source of knowledge in the biomedical domain. In this paper, we evaluated the coverage of UMLS as compared with Korean medical terms and identified differences in concept representation between two vocabulary sets. We measured the concept coverage by mapping clinical terms extracted from the discharge records of Seoul National University Hospital (SNUH) and the UMLS "Sign or Symptom" and "Disease or Syndrome" concepts. Thirty-five percent of the complaints in the SNUH were conceptually matched with the UMLS "Sign or Symptom" concept. Fifty-eight percent were found to be matched with the UMLS "Disease or Syndrome" concept rather than the "Sign or Symptom" concept. The remaining seven percent were not found in the UMLS concepts mapped above or those terms that used to special circumstances of a tertiary hospital in Korea. We then analyzed some of different expression patterns used by the two vocabulary sets and addressed issues to be taken into consideration.