We estimate the effect of subsidized childcare availability on Hungarian mothers’ labor supply, using a discontinuity in kindergarten eligibility at age 3 of children. The effect is identified in a setting where policy intervention has a high potential impact, since maternal labor supply is very low under age 3 of children, but high for mothers with older children. We find that access to subsidized childcare increases maternal labor supply by 11.7 percentage points or 24%, an impact that is higher than what has been found in previous quasi-experimental studies from most other countries. However, the potential effectiveness of future childcare expansion under age 3 may be constrained by further institutional factors, such as very long parental leave, traditional cultural views regarding maternal employment and institutional childcare, and the lack of flexible work forms. A comprehensive policy approach, including linked reforms of these other factors, is needed to achieve such a high effect.