Thirty-four children and adolescents with ages from 6 to 16 years having a diagnosis of attention-deficit hyperactivity disorder (ADHD)according to DSM-IV criteria were examined. Psychiatric comorbidity was evaluated through a standard questionnaire given to parents. There was a significant comorbidity (85,7%) in the sample, oppositional-defiant disorder (20,6%) and conduct disorder (39,2%) being the most common comorbid diagnosis. At least two diagnosis other than ADHD could be found in 57% of the sample. Major depression was seen in four cases and anxiety states (generalized anxiety disorder, separation anxiety and phobias) were seen in 34.3% of the sample. Comorbidity modifies prognosis in a significant way and may suggest specific therapeutic interventions according to each case. Frequent comorbidity may suggest the need for diagnostic interviews that covers other psychiatric disorders.