This study investigated whether the likelihood of motor impairment in children with attention-deficit/hyperactivity disorder (ADHD) increases with the presence of other disorders, and whether the co-occurring diagnoses of reading disability (RD) and oppositional defiant disorder (ODD) account for the motor deficits seen in ADHD. A total of 291 children (218 boys, 73 girls) participated. Six groups of children were compared: ADHD only (n = 29); RD only (n = 63); ADHD and RD (n = 47); ADHD and ODD (n = 19); ADHD, RD, and ODD (n = 21); and typically developing control children (n = 112). Motor skills were assessed with the Bruininks-Oseretsky Test of Motor Proficiency and the Beery Test of Visual-Motor Integration. We found that the motor skills of the ADHD-only group did not differ from the typical control group. Furthermore, motor impairment in ADHD increased as a function of co-occurring disorders, and the presence of RD rather than ADHD predicted motor impairment.