Increasing numbers of young adults are presenting in the primary care setting with signs and symptoms of attention-deficit/hyperactivity disorder (ADHD). Most of what is known about ADHD concerns children, but evidence has emerged over the past two decades showing that ADHD persists into adulthood more than 50% of the time. Presentation of the adult ADHD patient is different, requiring a flexible application of existing diagnostic criteria. Because the most effective treatment for ADHD remains the administration of schedule II stimulants, the diagnosis must be made with caution.