This study is the first to investigate the causal relation between universal childcare for three- to under-six-year-olds and long-term educational attainment in Germany using an instrumental variable approach. A reform, which legally entitled three-year-olds to childcare in 1996, caused an exogenous expansion of childcare supply, which serves as instrument. Using the German National Educational Panel Study (NEPS, 2010-2021, N = 7,886, 48% female; 22% immigrant origin; MeanAgeGraduation = 17.3), the results suggest that the reform extended children's childcare attendance up to 1.7 months for each 10 percentage points increase in supply, but a longer duration in childcare caused no attainment of higher school-leaving certificates. Findings illustrate the limited effectiveness of a mere quantitative expansion of universal childcare for enhanced educational attainment.