Primary schools in the Netherlands are autonomous in their decisions on grouping of pupils, and therefore on class size matters, because the state may not interfere in pedagogical-didactical issues according to the constitution. Surveys indicate that both across as well as within schools (across grades) class sizes consequently vary quite a lot. Capitalizing on this situation, a randomly drawn sample of 416 schools was used to collect data on class sizes and pupil achievement in grades 2, 4, 6 and 8. Social-ethnic status, IQ and gender also were examined. Using multi-level statistical models the strongest size effects were obtained in grade 2 in classes below 25 and above 35. Interaction effects between socio-ethnic status, sex and IQ with class size on achievement showed inconsistent trends. In three out of ten cases the achievement gap widened with decreasing class size.