Individual differences in a functional polymorphism of the promoter of the Monoamine oxidase A (MAO-A) gene might partly explain the increased vulnerability of maltreated children for externalizing behaviour. A sample of 239 internationally adopted boys was studied. Adoptive parents provided the information about abuse and neglect before the adoption and rated externalizing behaviour of their adopted children, using the Child Behaviour Checklist. MAO-A alleles were classified in high and low activity. We found that individuals with high MAO-A activity had more externalizing behaviour than those with low MAO-A activity. No modifying effect of MAO-A on the relationship between early maltreatment on externalizing behaviour was observed. Our results suggest that in severely maltreated children, high MAO-A activity may not protect against the effects of maltreatment but may convey an increased risk for externalizing behaviour. Psychiatr Genet 19:209-211 (C) 2009 Wolters Kluwer Health vertical bar Lippincott Williams & Wilkins.