We investigated correlations between measures of proactive and reactive aggression subtype, and the severity and frequency of overt aggression and psychiatric diagnosis in a clinically referred sample of children compared to a non-referred community comparison group free of psychiatric diagnosis. All measures of aggression were significantly correlated suggesting that there might be an underlying aggression construct that is manifested in distinct but correlated domains of aggression across diverse psychiatric diagnoses in referred children. Regression analysis revealed robust correlations between the number of lifetime psychiatric diagnoses adjusted for lifetime duration of illness and all measures of aggression. Our results support the need to consider the development of psychosocial and psychopharmacological treatment interventions specifically targeting excessive maladaptive aggression within and across multiple psychiatric diagnoses in children.