Objective: To describe weight, stature, and body mass index (BMI) changes occurring before the age of 7 years, which may influence the prevalence of overweight in adolescence and adulthood. Methods: Regression models predicting height and weight at ages 2 months to 6.75 years were based on the third National Health and Nutrition Examination Survey. Birth certificate data were used to adjust ethnic-specific models for birth weight for gestational age. Results: Attained height is higher for non-Hispanic black children than for either non-Hispanic white or Mexican American children (P ≤ .001). Weights at 2 months, 2 years, and 6.75 years are similar among racial/ethnic groups after adjustments. Mexican American children at ages 2 to 6.75 years had higher prevalences of BMI >85th percentile than either non-Hispanic white or black children (boys = 25.6%, SE = 2.7 compared with 14.1%, SE = 1.7 and 16.5%, SE = 1.7, respectively; girls = 21.9%, SE = 3.6 compared with 13.0%, SE = 1.7 and 13.7%, SE = 2.2, respectively). For non-Hispanic whites and Mexican Americans and for non-Hispanic black boys, BMI decreased slightly between ages 2 and 6.75 years; BMI for non-Hispanic black girls did not. Conclusion: Size differences before the age of 7 years may influence later ethnic-specific overweight prevalence, independent of prenatal influences. (J Pediatr 2000;137:205-13)