Children from low-income families benefit remarkably from exposure to compensatory education that began with Head Start in 1965 and aimed to improve school readiness skills by design. While empirical evidence has supported more instructional time in elementary and secondary schools for low-income students, little is known that whether increasing quantity of Head Start could also benefit low-income children. The authors' goal was to estimate the effect of Head Start hours on child development by examining: (1) Does the amount of daily exposure to Head Start impact cognitive, pre-academic, and social outcomes?; and (2) Does the impact vary by age? 4,442 applicants to 383 Head Start centers participated in the National Head Start Impact Study (HSIS). The results from the study showed significant positive effects of hours in center care experienced on the children's cognitive, language, and academic outcomes. Additionally, the results show that an additional hour per day spent in Head Start centers decreases problem behaviors of children in age-3 cohort, but results do not show significant effects for behavioral problems of age-4 cohort. Tables and figures are appended.