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.