A child with ADHD and a coexisting disruptive behavior disorder is likely to be similar to children with ADHD alone in terms of intelligence, medical history, and neurological development. He is probably no more impulsive than children with ADHD alone, although if he has conduct disorder, his teachers or other adults may misinterpret his aggressive behavior as ADHD-type impulsiveness. (Attention-deficit/hyperactivity disorder behavior without CD, however, does not typically involve this level of aggression.) A child with ADHD and CD does have a greater chance of experiencing learning disabilities such as reading disorders and verbal impairment. But what distinguishes children with ODD and CD most from children with ADHD alone is their defiant, resistant, even (in the case of CD) aggressive, cruel, or delinquent, behavior. Other indicators to look for include

Relatives with ADHD/ODD, ADHD/CD, depressive disorder or anxiety disorder. A child with family members with ADHD/ODD or ADHD/CD should be watched for ADHD/CD as well. Chances of developing CD are also greater if family members have experienced depressive, anxiety, or learning disorders. 

Stress or conflict in the family. Divorce, separation, substance abuse, parental criminal activity, or serious conflicts within the family are quite common among children with ADHD and coexisting ODD or CD. 

Poor or no positive response to the behavior therapy techniques at home and at school. If your child defies your instructions, violates time-out procedures, and otherwise refuses to cooperate with your use of appropriate behavior therapy techniques, and his aggressive behavior continues unabated, he should be evaluated for coexisting ODD or CD.