Research regarding attention deficit/hyperactivity disorder (AD/HD) among preschoolers is limited. This study explored prevalence rates of AD/HD on a community-based sample of preschoolers in Athens. Moreover, it examined the relationship between AD/HD and aggressive behaviour and explored sex differences in this relationship. Nursery teachers completed the 'Strengths and Difficulties Questionnaire' and the 'Aggressive Behaviour Questionnaire' for each one of the 925 boys and girls of the sample (mean age = 56.01 months, standard deviation = 8.9). Results revealed a prevalence rate of 14.3%, validating the existence of the disorder in preschoolers. Aggressive behaviour was positively correlated with AD/HD. Moreover, although girls without AD/HD presented less aggressive behaviour than boys, girls with AD/HD displayed certain forms of aggressive behaviour more often than boys with the disorder. Implications of these findings for early identification of AD/HD and for the use of aggressive behaviour in girls as a stronger indication of AD/HD than in boys are discussed.