Relationships among attention deficit hyperactivity disorder (ADHD) symptoms and adult personality traits have not been examined in larger clinically diagnosed samples. We collected multisource ADHD symptom and self-report NEO Five-Factor Inventory (Costa and McCrae [Odessa, FL: Psychological Assessment Resources, Inc, 1992) data from 117 adults with ADHD and tested symptom-trait associations using structural equation modeling. The final model fit the data. Inattention was positively associated with neuroticism and negatively associated with conscientiousness. On the basis of ADHD expression in adulthood, hyperactivity and impulsivity were estimated as separate constructs and showed differential relationships to extraversion and agreeableness. A significant positive relationship between hyperactivity and conscientiousness arose in the context of other pathways. ADHD symptoms are reliably associated with personality traits, suggesting a complex interplay across development that warrants prospective study into adulthood.