In order to achieve further insight into the comorbidity of reading disorder (RD) and attention deficit/hyperactivity disorder (ADHD), lexical processing and rapid naming were studied in RD and ADHD. The Dual Route Cascaded model postulates that lexical processing contains two parallel processes: lexical route processing and sublexical route processing. An orthographic decision task and a phonological decision task were used to measure lexical and sublexical route processing, respectively. In addition, a rapid naming task was used to compare 27 children with RD, 18 children with ADHD, 20 children with ADHD+RD, and 29 controls. RD and ADHD shared impairments in accuracy of orthographic and phonological decision making as well as in rapid naming, which suggest that RD and ADHD may be overlapping disorders that share deficits in both lexical route and sublexical route processing. RD was dissociated from ADHD by being slower in both orthographical and phonological decision making that indicates unique deficits in RD on lexical and sublexical speed. © 2012 Psychology Press.