Attendance in preschool centers can yield short-term benefits for children from poor or middle-class families. Yet debate persists in Europe and the United States over whether centers yield gains of sufficient magnitude to sustain children's cognitive or social advantages as they move through primary school. We report on child care and home environments of 229 children in the US who were 2 1/2 years of age (on average) at entry to the study. Among children attending a center at 2 1/2 or 4 1/2 years of age, cognitive proficiencies were significantly higher at 7 1/2 years of age, compared with children in home-based care, after taking into account prior proficiency levels, maternal attributes, and other covariates. No relationship between center attendance and social development, positive or negative, was detected at 7 1/2. A priori selection factors modestly helped to explain the likelihood that mothers enrolled their child in a center. But associations between center exposure and higher cognitive proficiency at age 7 1/2 remained after controlling for selection factors and testing for omitted variables bias. (Contains 5 tables, 3 figures, and 13 notes.)