Teacher questionnaires are used to assess children's social behaviour in a variety of contexts. Here questionnaire data are compared with observational data for agreement: (i) over sex and sibling status group differences; and (ii) over individual rank orders on more specific behavioural items. A principal component analysis of the teachers' ratings yielded four components--sociability, aggressiveness, perseverance and calmness. Boys were seen as more sociable and aggressive, girls more persevering. Firstborns were more sociable, aggressive and calm than secondborns, who were seen as more persevering than firstborns. Some differences between children with brothers and with sisters, and between children with same-sex and different-sex sibs were also found. These differences were in general in harmony with the observational data. Comparisons of questionnaire items with observational data on the same children indicated (a) that for the majority of items there was good agreement; (b) that agreement was better for the more behaviourally defined questions; (c) that teachers rated more on the children's behaviour with their peers than on their behaviour with the teachers; (d) that agreement was better at 42 months, soon after school entry, than at 50 months; and (e) that teachers' assessments may have been more influenced by the child's interactional style with themselves at the later age.