Comorbidity: a critical issue in attention deficit disorder.

In this report, we examine the interrelationships between attention deficit disorder (ADD), learning disabilities (LD), and conduct and oppositional disorders (COD). We indicate that it is reasonable to consider ADD as a distinct entity, frequently co-occurring with LD on the one hand, and COD on the other. The first section reviews the interrelationships between ADD and LD. Here we focus on definitional issues, trace the historical antecedents of ADD and LD, examine the prevalence of ADD and LD, and review studies designed to differentiate cognitive from attentional mechanisms in children with ADD, LD, or both. In the next section, we review the evidence linking ADD with COD, a distinction blurred in earlier investigations by problems with referral bias. More recent studies suggest that the antecedents, clinical characteristics, and prognosis may differ in children with ADD alone compared to those with ADD in association with COD.