Three states recently introduced universal prekindergarten programs offering free preschool to all age-eligible children; policy makers in many other states are promoting similar programs. Using restricted-access data from the Census, together with birthday-based eligibility cutoffs, I employ a regression discontinuity framework to estimate the effects of universal pre-K availability on overall preschool enrollment and maternal labor supply. Universal pre-K availability increases statewide preschool enrollment by about 14% but has little effect on the labor supply of most women. (c) 2010 by The University of Chicago. All rights reserved..