Objective: To assess the adequacy of 5 controlled medical terminologies (International Classification of Diseases 9, Clinical Modification [ICD9-CM]; Current Procedural Terminology 4 [CPT-4]; Systematized Nomenclature of Medicine, Clinical Terms [SNOMED-CT]; Logical Identifiers, Names, and Codes [LOINC]; Medical Entities Dictionary [MED]) for representing concepts in ophthalmology. Design: Noncomparative case series. Participants: Twenty complete ophthalmology case presentations were sequentially selected from a publicly available ophthalmology journal. Methods: Each of the 20 cases was parsed into discrete concepts, and each concept was classified along 2 axes: (1) diagnosis, finding, or procedure and (2) ophthalmic or medical concept. Electronic or paper browsers were used to assign a code for every concept in each of the 5 terminologies. Adequacy of assignment for each concept was scored on a 3-point scale. Findings from all 20 case presentations were combined and compared based on a coverage score, which was the average score for all concepts in that terminology. Main Outcome Measures: Adequacy of assignment for concepts in each terminology, based on a 3-point Likert scale (0, no match; 1, partial match; 2, complete match). Results: Cases were parsed into 1603 concepts. SNOMED-CT had the highest mean overall coverage score (1.625±0.667), followed by MED (0.974±0.764), LOINC (0.781±0.929), ICD9-CM (0.280±0.619), and CPT-4 (0.082±0.337). SNOMED-CT also had higher coverage scores than any of the other terminologies for concepts in the diagnosis, finding, and procedure categories. Average coverage scores for ophthalmic concepts were lower than those for medical concepts. Conclusions: Controlled terminologies are required for electronic representation of ophthalmology data. SNOMED-CT had significantly higher content coverage than any other terminology in this study. © 2005 by the American Academy of Ophthalmology.