Background: The behavioural and academic performance of young children with teachers' reported hyperactivity, conduct problems or inattention is under debate. Aim: This study investigates the associations between teachers' reported behavioural difficulties and academic and cognitive performances in two large samples of preschool and school children in France. Samples and method: Behavioural data relating to two large samples of preschool (N = 475) and first grade (N = 465) children were collected from their teachers by means of a questionnaire. A factorial analysis of the questionnaire revealed a four-factor structure ("hyperactivity," "inattention," "conduct problems" and "unsociability") that was similar in both age groups. Cognitive tests were used for each age group. Results: Teachers' reporting of "inattention" was associated with significantly lower performances in all tests in both the preschool and first grade samples. "Hyperactivity" or "conduct problems" were not consistently associated with the test results, when the effect of "inattention" was taken into account. Preschool "inattention," but not "hyperactivity" or "conduct/sociability problems," was related to poor performances at reading tasks in first grade. Conclusion: These findings question the pathological significance of teachers' report of "hyperactivity" in young children without associated attention problems.