The purpose of this study was to investigate changes in externalizing and internalizing problem behaviours in 6-year-olds with a focus on sex differences. Teachers rated problem behaviours at the beginning and at the end of the school year, 8 months apart, in 370 children (197 boys and 173 girls) attending 22 school preparatory classrooms. Although the majority of the children were quite stable, considerable negative and positive changes for both boys and girls in problem behaviours were found. The results showed that girls were more likely to change their externalizing behaviours in a positive direction than boys, whereas a tendency to the opposite pattern was found for internalizing behaviours. Boys were also found to be more prone to a negative change in problem behaviours of both types than girls were. (Contains 2 figures and 2 tables.)