The objective of the study is to evaluate the effectiveness and tolerability of atomoxetine in youth, ages 6-17 years with ADHD, who had a prior trial of stimulant treatment. This was a 6-week prospective open-label study of atomoxetine, dosage up to 1.4 mg/kg per day in 34 children and adolescents with DSM-IV ADHD. Primary measures of response included the ADHD Rating Scale (ADHD RS) and the Clinical Global Impression (CGI) Scale. Analyses were intention-to-treat. The treatment with atomoxetine was associated with statistical and clinical significant reduction in clinician rated ADHD RS symptoms, compared with baseline. Statistical significant improvement was attained by the second week of treatment. Fifty-six percent (N = 18) met criteria for our a priori definition of response; much or very much improved on the CGI plus more than 30% reduction in ADHD RS symptoms. In conclusion, atomoxetine was generally well tolerated; 85% of subjects completed the trial. Atomoxetine was effective and well tolerated in a 6-week open study of ADHD youth with a prior history of stimulant treatment.