Many adults with attention-deficit/hyperactivity disorder (ADHD) were never diagnosed as children. The impairment caused by untreated ADHD can complicate, or even lead to, other psychiatric conditions. Accurate diagnosis and efficacious treatment of ADHD in adults, which may include pharmacologic and nonpharmacologic interventions, is vital to improve their functioning. When a patient has ADHD and a co-occurring condition, the clinician should usually treat the most impairing condition first.