An open, naturalistic observational study design was used, in which a sample of seven boys between 10 and 14 years of age was assessed over 26 weeks. The subjects were all diagnosed with conduct disorder (CD) and exhibited high levels of aggression that were refractory to psychosocial approaches (parent counseling, school counseling, and psychotherapy) and to more than three pharmacological treatments. Levels of aggression, side effects, and hematological parameters were evaluated weekly. Clinical response was evaluated using the standardized instruments Clinical Global Impressions (CGI) and Child Behavior Check List (CBCL) 6-18 before starting medication and after 26 weeks. We observed good tolerability of clozapine in doses from 100 to 600 mg/day with no significant side effects or hematological changes. The CGI and CBCL 6-18 scales indicated that clozapine led to a marked control of symptoms.2344-8