The Unified Medical Language System (UMLS) was examined to determine its coverage of clinical laboratory terminology in use at the Columbia-Presbyterian Medical Center (CPMC). The Metathesaurus (Meta-1) contains exact matches for 30% of 1460 CPMC laboratory terms and near matches for an additional 42%, with better coverage of atomic-level concepts ("substance" terms) than complex ones (tests and panels). The Semantic Network includes types for representing laboratory procedures (2), measured substances (at least 56) and sampled substances (at least 14), but no type to represent specimens. Few of the UMLS semantic relationships are applicable to the CPMC vocabulary. These results have implications for the utility of the UMLS for linking clinical databases to electronic medical information sources.