As the number of schools changing from part- to full-day kindergarten programs increases, state and local education agencies need empirically-based evidence on ways that schools and teachers can best structure the additional instructional time of full-day programs to improve children's early reading skills. This brief uses nationally representative data from the Early Childhood Longitudinal Study, Kindergarten Class of 1998-99 (ECLS-K) to explore relationships between full-day kindergarten program factors and public school children's gains in reading scores from the fall to spring of the kindergarten year. Results from the study provide evidence that: (1) Children in kindergarten programs that devote a larger portion of the school day to academic instruction, and to reading instruction in particular, make greater gains in reading over the school year than children who spend less time in such instruction; (2) Children tend to make optimal gains in reading when teachers use an equal balance of discrete literacy skills and comprehension skills instruction; and (3) Class size interacts significantly with some instructional practices to increase or decrease children's average reading gains in kindergarten. In summary, this brief provides some of the first evidence on how full-day kindergarten programs might structure instructional resources and practices in ways that prepare children for first grade and later school success. (Contains 3 figures and 4 endnotes.) [This Research Brief is based on work conducted for the dissertation, "Making the Most of Extra Time: The Role of Classroom Factors and Family Socioeconomic Status on Full-Day Kindergartners' Reading Achievement and Academic Engagement" (Rathbun, 2007).]