Objective: To compare the reduction of ADHD symptoms under atomoxetine (ATX) in patients with and without pretreatment with a stimulant medication using a computer-based Continuous Performance Test (cb-CPT) combined with an infrared motion tracking (MT) device. Method: Double-blind, placebo-controlled study in ADHD patients (6-12 years) treated with ATX (target dose = 1.2 mg/kg per day). The cb-CPT/MT scores were analyzed using ANCOVA (last observation carried forward). Results: Patient data (n = 125) suggested a differential ATX treatment effect between pretreated and stimulant-naïve patients in terms of three cb-CPT/MT parameters. Conclusion: This secondary analysis provided evidence that ATX reduced ADHD symptom severity measured by cb-CPT/MT parameters regardless of stimulant pretreatment. A few differential effects were seen based on the cb-CPT/MT. However, no clear pattern could be identified and, overall, the observed differences have no larger clinical relevance. The ATX effect in this study seemed to be largely independent of any previous exposure to stimulants. (J. of Att. Dis. 2012; XX(X) 1-XX)