The study objective was to assess the efficacy and tolerability of atomoxetine in the treatment of attention deficit hyperactivity disorder symptoms in patients with mental retardation. In a 16-week, open-label, prospective study, 48 children with mental retardation and attention deficit hyperactivity disorder were recruited; the patients received atomoxetine, with a single final dose of 1.2 mg/kg per day reached at 3 weeks. The measure of efficacy was scores on Clinical Global Impression Severity scale (CGI-S), Conners, and Attention Deficit Hyperactivity Disorder Rating Scale ADHDRS-IV. A statistically significant difference was documented between the mean CGI-S scores before and after treatment: baseline CGI-S = 5.31 (S.D. = 0.85); post-treatment CGI-S = 4.13 (S.D. = 0.97), with a difference of 1.18 points (S.D. = 0.84) and a 95% confidence interval for the difference of 0.92-1.43 ( P < 0.001). A statistically significant reduction ( P < 0.01) was observed with respect to all the variables of the ADHDRS-IV and Conners scales. Slightly less than one third of the patients (31%) presented adverse events, the majority of which were mild, with irritability being the most frequent event. Atomoxetine appears to be to useful in improving attention deficit hyperactivity disorder symptoms in mentally retarded patients. Larger, randomized, controlled, double-blind studies are required to confirm the efficacy observed in this first study.