Substantial evidence suggests that young children reason about illnesses using biological causal principles and that, with increasing age, children provide more explicit accounts of mechanisms underlying different conditions. To investigate these claims, sixteen 7-8 year-old children and sixteen 11-12 year-old children with attention deficit-hyperactivity disorder (ADHD) completed an open-ended interview about the etiology and treatment of ADHD, aggressive behavior, and colds. Younger children used either biological or psychological principles to explain ADHD, while older children used biological causality or integrated biological and psychological principles into explanations. Children at both ages used biological causality to explain colds, and intentional, psychological causality to explain aggression. Older children provided more complex accounts of the mechanisms underlying conditions. The findings suggest that school-aged children utilize principles spanning different domains to explain conditions with physical and behavioral symptoms. Implications for domain-specific accounts of cognitive development and for the care of children with ADHD are discussed.