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Measurements of mesiodistal crown size diameters were obtained from dental casts of 114 Down syndrome subjects. These measurements were compared to measurements similarly obtained from a clinically normal subsample of participants in the National Collaborative Perinatal Project. II, 12, dc, dml, dm2, and Ml were measured. Results demonstrated that mesiodistal diameters of dc and dml were significantly greater in Down syndrome than in the normal controls. Dm2 was not significantly different in size from normal. II, 12, and Ml were generally found to be significantly smaller than normal in Down syndrome subjects. Down syndrome subjects may experience an initial transitory acceleration in the mitotic activity of enamel organs during a period of early development of the deciduous dentition, followed by the widely recognized retardation in growth which is apparent in the permanent dentition and in most other attributes of growth in Down syndrome.