In 1975 a longitudinal study was begun to determine the impact of the New York State Experimental Prekindergarten Program on the children's cognitive development. The program enrolls approximately 6,500 children annually. Three cognitive measures were used to evaluate program effects: the Walker Readiness Test for Disadvantaged Children, the Cooperative Preschool Inventory, and the Peabody Picture Vocabulary Test. First-year results indicated that the program had a positive effect on children's performance on the Walker and the Peabody. During the second year, children were studied in kindergarten and in prekindergarten. Results indicate that: (1) the positive effects of the program were still evident on the Walker and the Cooperative at the beginning of kindergarten; (2) effects on the Walker persisted at least until the end of kindergarten; (3) the program was most effective for children whose mothers have the least education and perhaps for children whose families have the lowest income; and (4) the program had a positive effect on the performance of a second wave of children on all three measures administered at the end of prekindergarten. (Author/CTM)