This dissertation report describes a 3-year study which compared the effectiveness of full-day kindergarten sessions (the experimental condition) and half-day sessions (the control condition) in promoting academic achievement. A group of 20 5-year-olds was assigned to each condition; each group contained equal numbers of culturally disadvantaged, economically disadvantaged and middle-class children. Readiness scores and achievement test scores for children in both groups were obtained at the beginning and end of the kindergarten year. Subsequent first grade placement and reading level attainments at the end of first grade were recorded. Results indicated that there was no statistically significant difference between full-day and half-day groups on any of these measures. When achievement test scores of the culturally disadvantaged were analyzed alone, there was still no statistically significant gain in achievement by the full-day over the half-day, indicating that the full-day program did not provide any special academic benefit to culturally disadvantaged children. It is suggested that on the basis of these results, the establishment of full-day kindergarten cannot be recommended on the grounds that it would measurably improve children's achievement. (Author/GO)