Parental education and mothers education in particular are believed to play an important role for children’s academic outcomes. This paper analyzes the causal effect of mothers' education on children’s academic performance. Using Danish administrative data on mothers’ education careers, we apply a difference-in-differences design to control for the nonrandom selection into maternal education and find that mothers’ education has a positive effect on children’s academic performance. The effect diminishes, the higher the mothers’ initial educational attainment.