The psychiatric histories of relatives of 17 boys admitted to an inpatient unit with "unsocialized aggressive" behavior and of 19 boys admitted with other problems were investigated by means of structured interviews. The interviews were rated blind and diagnoses made on the respondents and relatives described in the family histories using research criteria. Fathers and combined parents of the probands qualified for the diagnosis of antisocial personality disorder more often than the fathers and combined parents of control boys. Uncles of probands and combined uncles and aunts more often had a history of marked antisocial behavior than the corresponding relatives of controls. Siblings of the probands more often had a history of behavior problems as children or adolescents than the siblings of controls. The data suggest an association between unsocialized aggressive reaction in boys and antisocial personality disorder in their adult relatives. Such an association is an interesting contrast to the relationship between hyperactivity in children and alcoholism in their adult relatives.