The purpose of the study was to investigate how grading of students in primary school affected achievement measured by grades one year later, and how the effect varied as a function of cognitive ability, gender and socio-economic status. The data derive from The Evaluation Through Follow Up longitudinal project. Through a national curriculum change one group of students received grades and one group of students did not receive grades in the 6th Grade, while both groups of students received grades in the 7th Grade. A quasi-experimental design was thus created, with two comparable groups of students. The subjects were 8558 students born in 1967. Several multiple regression models were fitted to the data. The results showed no main effects of grading on subsequent achievement, but there were important differential effects: graded students with low cognitive ability received lower subsequent grades. The results also showed interaction effects between gender and grading and between gender and cognitive ability.