We estimated the effects of an experimental educational day care program on the intellectual development of preschoolers from 86 high-risk families in a randomly allocated longitudinal study. At six through 54 months of age, the IQs of experimental program children ranged from 7.9 to 20.1 points higher than those of control children when maternal mental retardation and home environment effects were controlled; at every age, a greater proportion of the experimental program children had normal range IQs (greater than 84). In 13 children with retarded mothers, none of six experimental program children, but six of seven control children, had IQ scores below normal.