Objective: The purpose of this 13-study (seven double-blind/placebo-controlled, six open-label) meta-analysis is to determine the effectiveness and tolerability of long-term atomoxetine treatment among young children with attention-deficit/hyperactivity disorder (ADHD). Method: Data were pooled from 6- and 7-year-olds (N = 272) who met DSM-IV criteria for ADHD, received atomoxetine treatment, and were enrolled in clinical trials of greater than or equal to 2 years. Of these, 97 subjects reached the 24-month time point, providing data for long-term trend analysis of safety and effectiveness. Results: Effectiveness for most subjects was maintained over long-term treatment, as demonstrated by total scores and total T scores on the Attention-Deficit/Hyperactivity Disorder Rating Scale-IV-Parent Version, investigator administered and scored. During the 2-year period, 25.7% discontinued because of lack of effectiveness, but adverse events were clinically minor and transient, and only 4.0% of children discontinued because of an adverse event. Notable effects on growth were seen during early phases of the study, with attenuation occurring by the 2-year time point. Statistically significant increases in pulse and blood pressure and decreases in cardiac PR interval were seen, but no changes were deemed both statistically significant and clinically meaningful among any vital signs, electrocardiographic measures, or laboratory tests. Conclusion: Long-term atomoxetine treatment appears generally well tolerated and effective in the treatment of young children with ADHD. (Contains 4 tables and 1 figure.)