The Behaviour Style Observation System for Young Children (BSOS) was used to predict preschool-aged children's externalizing and internalizing behaviour problems in middle childhood, 3-5 years after the initial assessment. This observational measurement tool was designed to sample and assess young children's disruptive, non-compliant, and unresponsive behaviour, during a brief (11 min) observation in the child's home. In the current study, the BSOS was used to predict parent and teacher ratings of child behaviour problems after school entry in a longitudinal sample (N = 81) of at-risk children at time 2. The BSOS predicted teacher-reported externalizing problems at time 2. In contrast, parent reports of behaviour problems, although correlated with repeated parent reports at time 2, were not significantly predictive of teacher-reported behaviour problems at school age. The BSOS was not associated with either parent or teacher reports of internalizing problems. These findings emphasize the importance and utility of using observational measures when examining the continuity of behaviour problems in young children over time. (Contains 5 tables.)