To better understand the implications of comorbidity between reading disability (RD) and attention-deficit/hyperactivity disorder (ADHD), a sample of 225 participants with RD but without ADHD, 139 participants with both RD and ADHD, and 1,502 children without reading or attentional difficulties was recruited through five large public school districts. In comparison to the group without RD or ADHD, both groups with RD exhibited elevations of comorbid internalizing and externalizing disorders and significant global, academic, and social impairment. However, the group with both RD and ADHD was most impaired on most measures, and analyses of neuropsychological measures indicate that the co-occurrence of RD and ADHD may be due at least in part to weaknesses in cognitive processing speed and working memory that are most severe in the comorbid group. These results indicate that psychoeducational assessments of RD should always screen for ADHD and other emotional and behavioral difficulties, and that when RD and ADHD co-occur interventions are likely to be needed for both disorders.