The relationship between behaviour problems and intelligence was examined in a sample of 106 three-year-old boys and girls selected for the presence of either conduct problems or hyperactivity. Parents' reports of hyperactivity, but not conduct problems, were negatively related to IQ, scores for both boys and girls. There was a significant sex difference in the association between conduct problems and IQ. For girls IQ was positively correlated with conduct problems. The implications of the results for existing models of the relationship between behavioural deviance and cognitive functioning in childhood is discussed.