THE OBJECTIVE OF THIS STUDY WAS TO MINIMIZE THE VARIABILITY IN LEGAL AND EDUCATIONAL CLASSIFICATION OF THE MENTALLY RETARDED. A SYSTEMATIC REVIEW WAS MADE OF THE SEMANTIC VARIATIONS IN THE DEFINITION OF THE MENTALLY RETARDED IN TERMS OF FUNCTIONAL ADEQUACY FOR EDUCATION, TRAINING, OR CUSTODIAL CARE. THE METHOD INVOLVED THE REVIEW OF STATUTES AND REGULATIONS ABOUT THE CLASSIFICATION AND EDUCATION OF THE MENTALLY RETARDED, A SURVEY OF PROCEDURES USED IN THEIR CLASSIFICATION, AND AN EXAMINATION OF PUBLISHED SOURCES FOR NAMES USED IN THEIR CLASSIFICATION. ON THE BASIS OF THE PRESENT REVIEW OF LITERATURE, A TENTATIVE MULTIDIMENSIONAL SYSTEM FOR THE DEFINITION OF TERMS HAS BEEN DEVELOPED. THE PROPOSED SYSTEM CONSISTS OF A NUMBER OF DIMENSIONS CONCERNED WITH ETIOLOGY FUNCTIONING, STATUS, AND PROGNOSIS. EACH DIMENSION IS DIVIDED INTO SEVERAL SUBCATEGORIES, REPRESENTING EITHER QUALITATIVE DIFFERENCES, AS IN ETIOLOGY, OR QUANTITATIVE DIFFERENCES. THE DEVELOPMENT OF THIS SYSTEM WOULD PROVIDE SOME STANDARDIZED SET OF REFERENCES FOR DEFINING TERMS. IT IS UNLIKELY THAT ONE SET OF TERMS WILL BE ACCEPTED AND FOUND TO BE GENERALLY USEFUL IN THE VARIETY OF SETTINGS IN WHICH PROBLEMS OF MENTAL RETARDATION ARE OF CONCERN, BUT A STANDARD SYSTEM OF DEFINING TERMS WOULD PROVIDE A COMMON BASIS FOR COMMUNICATION AND TRANSLATION FROM ONE SET OF USAGES TO ANOTHER. FURTHER RESEARCH MIGHT INVESTIGATE THE INTERRELATIONSHIPS AMONG THE VARIOUS DIMENSIONS OR MIGHT BE DIRECTED AT METHODS OF SUMMARIZING THE DESCRIPTIVE DATA, USING VARIOUS SCORES, CUT-OFF POINTS, OR PROFILES, AND RELATING THIS INFORMATION TO EDUCATIONAL AND THERAPEUTIC GOALS. (JL)