DATA OBTAINED FROM INTERVIEWS BASED ON QUESTIONNAIRES WERE USED TO ASSESS LEVEL OF AGGRESSIVENESS AND DISCOVER THE DETERMINANTS OF AGGRESSIVENESS IN THE FAMILY BACKGROUND. THE SUBJECTS' ANSWERS TO EACH OF THE QUESTIONS WERE MARKED ON A FIVE-POINT RATING SCALE. ANOTHER QUESTIONNAIRE WAS USED TO CHECK THE ASSESSMENTS OF AGGRESSIVENESS. THE LEVEL OF AGGRESSIVENESS WAS RATED BY FREQUENCY OF AGGRESSIVE REACTIONS WHICH WERE DISPROPORTIONATE IN STRENGTH TO THE STIMULUS, BY HOSTILITY, AND BY INABILITY TO CONTROL AGGRESSION. IN THE OFFENDER GROUP, THE NUMBER OF SUBJECTS RATED AS VERY AGGRESSIVE WAS 37 SIGNIFICANTLY GREATER THAN THE CORRESPONDING NUMBER OF NONOFFENDERS, WHICH WAS 8. HOSTILITY WAS FOUND IN BOTH AGGRESSIVE OFFENDERS AND AGGRESSIVE NONOFFENDERS, ALTHOUGH TO A LESSER DEGREE FOR THE LATTER. THE INTERVIEW DATA ALSO SHOWED THAT AMONG AGGRESSIVE OFFENDERS MANY FORMS OF AGGRESSION APPEARED, DIRECTED AGAINST A GREATER NUMBER OF PEOPLE THAN AMONG THE NONOFFENDERS. RATING FAMILY BACKGROUND DETERMINANTS OF AGGRESSIVENESS SHOWED THAT FATHERS AS MODELS OF AGGRESSION, AND THAT A DISTURBED EMOTIONAL RELATIONSHIP BETWEEN FATHER AND SON, AND A MOTHER'S TOLERANCE OF THE SON'S AGGRESSION FOSTER AGGRESSIVE BEHAVIOR. EXTENSIVE THEORETICAL BACKGROUND, TABULAR DATA, THE QUESTIONNAIRES USED IN THE STUDY, AND BIBLIOGRAPHICAL REFERENCES ARE INCLUDED.