We aimed to investigate the association between attention deficit hyperactivity disorder (ADHD) and morningness–eveningness in adulthood. Subjects without psychiatric comorbidity on the Structured Clinical Interview for DSM-IV Axis I Disorders (n = 344) completed the Morningness–Eveningness Questionnaire (MEQ) and the Adult Self-Report Scale for ADHD. MEQ showed an independent and negative association with ADHD symptoms (P < 0.0001). In male subjects, both inattention (P < 0.0001) and hyperactivity–impulsivity were associated with MEQ (P = 0.01). In female subjects, only inattention was associated with MEQ (P < 0.001). Our findings suggest that eveningness may be strongly associated with inattention of adult ADHD and that it may be associated with hyperactivity–impulsivity of adult ADHD in male subjects only.