Background: To evaluate the effect of atomoxetine (ATX) on attention-deficit/hyperactivity disorder (ADHD) and comorbid social anxiety disorder in adults. Methods: Randomized, double-blind, placebo-controlled, conducted in adults with ADHD and social anxiety disorder. Patients received 40–100 mg ATX (n=224) or placebo (n=218) for 14 weeks following a 2-week placebo lead-in period. Efficacy measures included the Conners' Adult ADHD Rating Scale: Investigator-Rated: Screening Version (CAARS:Inv:SV), Liebowitz Social Anxiety Scale (LSAS), Clinical Global Impression-Overall-Severity (CGI-O-S), State-Trait Anxiety Inventory (STAI), Social Adjustment Scale-Self Report (SAS), and Adult ADHD Quality of Life Scale-29 (AAQoL). Safety and tolerability were also assessed. Results: ATX mean change (−8.7±10.0) from baseline (29.6±10.4) on CAARS:Inv:SV Total ADHD Symptoms score was significantly greater than placebo mean change (−5.6±10.2) from baseline (31.2±9.4; P<.001). ATX mean change (−22.9±25.3) from baseline (85.3±23.6) on LSAS Total score was significant compared to placebo mean change (−14.4±20.3) from baseline (82.1±21.3; P<.001). The visit-wise analysis revealed greater improvement on the CAARS:Inv:SV Total ADHD Symptoms score and LSAS Total score for ATX at every time point throughout the study (P values ≤.012). Mean changes in CGI-O-S, STAI-Trait Anxiety scores, and AAQoL Total score were significantly greater for ATX compared to placebo. Mean change for both groups on STAI-State Anxiety scores was comparable. Improvement on SAS for ATX compared to placebo was not significant. Rates of insomnia, nausea, dry mouth, and dizziness were higher with ATX than with placebo. Discontinuation rates due to treatment-emergent adverse events were similar between groups. Conclusions: ATX monotherapy effectively improved symptoms of ADHD and comorbid social anxiety disorder in adults and was well tolerated. Depression and Anxiety, 2009. Published 2009 Wiley-Liss, Inc.