This paper investigates the role of preschool quality for children's school performance at the end of primary school. We construct five structural quality indicators based on unique Danish administrative register data. 30,444 children finishing primary school's 9th grade in 2008 and who attended a formal preschool institution in 1998 are used in the analyses. OLS analyses show that three out of five quality indicators, a higher staff-per-child ratio, a higher share of male staff, and a higher share of staff with formal preschool teacher training are associated with significant improvements in children's test results in Danish. Boys benefit more from preschool quality than girls. Ethnic minority children benefit from higher staff stability. (author abstract)