Nearly all school-age children in the United States attend kindergarten, and approximately three-quarters of kindergarten students are in full-day classrooms. While there have been dramatic increases in provision of and participation in full-day kindergarten, there is little evidence on the impact and cost-effectiveness of such programs and policies, particularly as compared to other types of investments in early childhood. Employing data from districts assigning students to kindergarten settings by lottery, I test the impact of full- versus half-day assignment on students’ literacy skills at the end of the kindergarten year, generating the first evidence based on random assignment of children to kindergarten type. The results indicate that full-day assignment has a substantial, positive effect (0.31 standard deviations) when comparing students across treatment conditions within the same school. In particular, I find that Hispanic students realize large full-day kindergarten effects (0.70 s.d.), and notably this impact is statistically different than that experienced by students who are not Hispanic. Students who enter kindergarten with low literacy skills also experience particularly large gains. These heterogeneous treatment effects have implications for narrowing or closing the achievement gap early in formal schooling, and in fact the impact for Hispanic students constitutes approximately 70 percent of the control group’s end-of-kindergarten ethnicity gap. Using rough cost measures, a simple cost-effectiveness analysis suggests a range of effect sizes from 0.07–0.20 s.d. per thousand dollars of spending, which exceeds similar calculations from experimental evidence on other early investments. Given the positive evidence on program effects, I discuss implications of the study design and findings for policy, including targeted versus universal provision of full-day kindergarten.