OBJECTIVE: Morningness is a stable characteristic of individuals, related to impulsivity and novelty seeking. The evening orientation is a risk factor for psychiatric conditions such as depression and personality disorders. The authors hypothesized that adults suspected of having ADHD are more evening oriented than adults without ADHD. METHOD: Exploratory factor analyses were performed on the polychoric correlation matrices of the full Adult Self-Report Scale for ADHD (ASRS) and the Composite Scale of Morningness (CSM). RESULTS: As expected, two factors for the ASRS--Inattention and Impulsivity- Hyperactivity--and three factors for the CSM were retrieved. All three CSM factors correlated negatively with ASRS Inattention, none with ASRS Impulsivity-Hyperactivity. CONCLUSION: Inattention is more strongly related to eveningness than is Impulsivity-Hyperactivity, and therefore eveningness may constitute an endophenotype of the predominantly inattentive subtype of ADHD. These findings need to be replicated in a patient population with a standardized assessment of sleep quality.