The frequencies of patients with low thrombocyte monoamine oxidase (MAO) activity (defined as having an activity lower than 1 SD below the mean of a respective control group) were studied in 100 consecutive cases admitted to a clinic for child and youth psychiatry. 41 boys and 26 girls (group I) had behavior disorders, attention deficit disorders and/or alcohol and hashish abuse according to DSM III R. 16 boys and 17 girls had other diagnoses (group II). None of the 2 male patient groups differed significantly from the controls. A significantly higher percentage of the girls in group I had low activity of platelet MAO than in the control group (p = 0.015), while the girls in group II did not differ from the controls. Acting out, antisocial behavior and abuse is less accepted in women than in men. Therefore, the girls in the present study might be either more psychiatrically disturbed or have more deviant personalities than the boys, which might explain why our hypothesis about a lower thrombocyte MAO activity in the adolescents with externalizing symptoms (group I) was verified only in the girls.