Purpose of the article: Cognitive training for Attention Deficit/Hyperactivity Disorder (ADHD) has shown promising, although mixed results. In post-hoc analyses, we evaluate effects of cognitive training using a novel composite cognition score as the outcome for children attending at least 16 sessions of training, dose-response of training and associations between symptoms and cognitive functioning. Materials and methods: Children (age 6–13) with ADHD were randomized to intervention (n = 26) or control (n = 34). For the current analysis, we restricted the intervention group to children, who completed at least 16 sessions of cognitive training (n = 26) and examined a dose response within that group. Results: Cognition improved significantly in the intervention, but not control group. Amount of the completed training sessions correlated significantly with the amount of cognitive improvement. Conclusion: Variations in dose and frequency of training may be an important source of the variance in previous studies.