In the 1980s and 1990s, many states and districts began to provide funding for prekindergarten (PK) programs for the first time. This paper takes advantage of the staggered timing in program funding to investigate the effect that increased availability of PK programs has on the labor supply of mothers with 4-year-olds. I find that mothers with a 4-year-old and no younger children were significantly more likely to be in the labor force and employed once PK became available. Mothers with a 4-year-old and other younger children were also significantly more likely to be in the labor force and employed. (author abstract)