Background: The objective of this study was to estimate the prevalence of Seasonal Affective Disorder (SAD) in adults with lifetime Attention-Deficit/ Hyperactivity Disorder (ADHD). Method: Patients eligible for this study had lifetime impairing symptoms of ADHD and a current and/or past co-morbid mood disorder according to their medical record. The Seasonal Pattern Assessment Questionnaire (SPAQ) was administered by a telephone interview to assess seasonality. Results: The overall rate of SAD in this clinical population of adults with ADHD was estimated at 27%. Females were more at risk to develop SAD than men. Limitations: The SPAQ is a screening, not a diagnostic instrument. Conclusions: SAD symptoms are frequently comorbid with ADHD in adults. These results have clinical relevance for the recognition and treatment of SAD with bright light therapy in adults with ADHD. © 2005 Elsevier B.V. All rights reserved.