This study assessed the efficacy of a 5-week, intensive working memory training program for 52 children and adolescents (ages 7-17) who had Attention-Deficit/Hyperactivity Disorder (ADHD) and other comorbid diagnoses. This study provided a treatment replication since the waitlist control group also completed training and was included in the follow-up data analyses. Parents and teachers completed paper-and-pencil measures of working memory, executive functioning, and ADHD symptoms at baseline, posttreatment, and 4-month follow-up. Parent ratings indicated that participants improved on inattention, overall number of ADHD symptoms, initiation, planning/organization, and working memory. Teacher ratings approached significance at posttreatment and at 4-month follow-up on and Initiate scale. Working memory training appears promising as an intervention in improving executive functioning and ADHD symptoms.