Gender differences in educational achievement were examined in a cohort of 1265 individuals studied from birth to age 25. There was a small but pervasive tendency for females to score better than males on standardised tests and to achieve more school and post-school qualifications. The differences could not be explained by differences in cognitive ability as males and females had similar IQ scores. Teacher ratings of classroom behaviour revealed that males were more prone to inattentive, restless and distractible behaviours and aggressive, antisocial and oppositional behaviours than females. When the associations between gender and measures of educational achievement were adjusted for teacher ratings of classroom behaviour the gender differences were reduced substantially. These results suggest that one approach to reducing gender differences in educational achievement lies in improving classroom behaviour. (Contains 4 tables.)