Approximately 6,500 children are enrolled per year in the New York State Experimental Prekindergarten Program (PreK). In 1975-76 a longitudinal study was begun to determine the program's impact on the development of children while enrolled in the program and in the primary grades. Samples from a population of 5,245 four-year-old prekindergarten children and a control group of 87 children were studied. Data were collected on children's cognitive functioning, social and classroom competencies, personal and family characteristics, school attendance, participation of parents in the prekindergarten program, noninstructional services, and general program characteristics. Analyses of covariance were carried out on three tests of cognitive development with one of the following variables: child's age at pretesting, mother's education, family income, months between pretest and post test, and pretest scores on measure for which post test scores were used as a post test and pretest scores on measure for which post test scores were used as a criterion. A significant positive program effect was found for two of the cognitive measures, the Cooperative Preschool Inventory and the Walker Readiness Test. Analyses of the Peabody Picture Vocabulary Test scores did not show a significant PreK effect, even in the presence of the control variables. (Author/MV)