The primary objective of this study was to examine the effects of the Cogmed training program on working memory among youths 7 to 13 years old, with attention deficit/hyperactivity disorder (ADHD) type and comorbidity controlled for. A secondary objective was to examine the generalization of effects to ADHD symptoms, nonverbal reasoning, attentional and executive functions, inhibition, reading comprehension, and mathematical reasoning. Participants were under pharmacological treatment for ADHD combined type and a comorbidity. They were randomized into an experimental group that received the Cogmed program and an active control group that received a low-intensity comparison version of the training. They were evaluated at three time points: 6 weeks prior to intervention onset (T1), immediately prior to onset (T2), and 1 week following intervention completion (T3). Results indicate no significant effect attributable to the Cogmed program. The fact that participants were on medication at the time of training and evaluation normalized their performances and limited the detection of effects. Moreover, cognitive training did not lead to a reduction in ADHD symptoms or to an improvement in the other cognitive functions measured or in academic performance. The results of this study do not demonstrate the effectiveness of the Cogmed program for youths with ADHD combined type and a comorbidity when they receive the training while under pharmacological treatment.