Objective: The aim of this study was to examine the effects of Cogmed training on working memory in adults with ADHD, including the generalization of effects to nonverbal reasoning, executive function in daily life, and ADHD symptoms. Method: Adults of ages 18 to 63 with ADHD were randomly assigned to an experimental group that received the working memory training or to a placebo group that received a low-intensity comparison version of the training. Results: participants in the experimental group showed greater improvement in verbal and visuospatial working memory capacity. Effects persisted 6 months after intervention completion. However, effects did not generalize elsewhere. Conclusion: The results challenge the claims made by Cogmed to the effect that the program is effective in treating attention problems and improving performance in daily life over the long term in adults with ADHD.