Longitudinal mental test scores for 54 educationally treated disadvantaged preschool children at high-risk for nonbiologically based mild mental retardation and 53 control children were compared. The educationally treated children were in a child-centered prevention-oriented intervention program delivered in a daycare setting from infancy to age 5. Language, cognitive, perceptual-motor, and social development were stressed. Children were examined with age-appropriate tests of development at 6, 12, 18, 24, 30, 42, 48, and 54 months of age. Beginning at 18 months, and on every test occasion thereafter, educationally treated children significantly outscored control group children on mental tests; treated children consistently scored at the national average whereas control children's scores declined from the average level at 12 months to below average at 18 months and thereafter. Implications of the results for early intervention were discussed.