Difficulties in phonological processing and reading that characterize developmental dyslexia have been suggested also to affect those with attention-deficit/hyperactivity disorder (ADHD). However, it is not known to what extent various intervening factors, such as low intelligence quotient or age, explain the observed difficulties. In this study, the authors investigated the domains of phonological processing, reading, spelling, and arithmetic in 110 adults (ages 18-55 years) in healthy control, dyslexia, and ADHD groups. The aim of the study was specifically to compare domain profiles of participants with ADHD to those in other groups. The results showed that participants with dyslexia had the most generalized difficulties. Participants with ADHD were the least affected, and their difficulties reflected less accurate performance. Furthermore, all the observed differences became nonsignificant when intelligence quotient was controlled for. This suggests that adult ADHD is not related to significant impairments in phonological processing or achievement. (Contains 3 tables and 1 figure.)