Although most medical lexicons contain up to 80% of clinical terms used in an ambulatory patient medical records archive, preliminary research suggests that they may be far less complete for radiology terms. We therefore compared the likelihood that several existing medical lexicons would contain terms found in a radiology report to the likelihood they would contain terms found in an ambulatory care medical record. We used three samples of imaging terms to assess the completeness of existing lexicons for medical imaging: (1) a random sample of imaging terms from the Unified Medical Language System Large Scale Vocabulary Test (UMLS-LSVT; n = 218), (2) terms actually used in the first 80 clinical knee magnetic resonance imaging reports generated by the routine clinical use of a structured reporting system (eDictation, Marlton, NJ; n = 76), and (3) terms listed in a glossary of thoracic imaging prepared by the Fleischner Society (n = 173). Using the UMLS Web-based Knowledge Source Server (http://umlsks.nlm.nih. gov/), we measured the rate at which terms in each of the above three sources were found in the UMLS and two of its major constituent terminologies: ICD-9-CM and SNOMED International. ICD-9-CM contained matches for 3%, 8%, and 11% of terms from the Fleischner Society Glossary, eDictation, and NLM-LSVT, respectively. SNOMED International contained matches for 32%, 46%, and 36% of terms from the Fleischner Society Glossary, eDictation, and NLM-LSVT, respectively. The UMLS contained matches for 36%, 50%, and 45% of terms from the Fleischner Society Glossary, eDictation, and NLM-LSVT, respectively. The assessed vocabularies were least likely to contain a term from the Fleischner Society Glossary and most likely to contain a term from the eDictation lexicon. The UMLS was the most complete, and ICD-9 was the least complete of the three systems evaluated. No lexicon achieved greater than 50% completeness for any test set of imaging terms. Our results show that no single lexicon is sufficiently complete to allow comprehensive indexing, search, and retrieval of radiology report information. These results confirm the few results available from the medical literature indicating that existing controlled vocabularies are insufficiently complete to represent the contents of radiology reports. A subjective analysis of these results may identify particular imaging sub-areas for which new terms should be developed.