Background: Attention-deficit hyperactivity disorder (ADHD) is a common psychiatric disease, usually first diagnosed during childhood or adolescence. A significant portion (∼50–65%) of ADHD symptoms persist into adulthood. Adult ADHD and bipolar disorder (BD) are common comorbid psychiatric conditions. Comorbidity between BD and adult ADHD has been reported in both ADHD and BD cases. Aim To determine the effect of comorbid adult ADHD on disability and quality of life of patients with bipolar disorder. Patients and methods This is a cross-sectional study that was done at the Psychiatric Department in the Zagazig University Hospitals, Sharkia Governorate, Egypt, during the period from May to November, 2017. One hundred and two euthymic patients with Diagnostic and statistical manual of mental disorders, 4th ed. − text revision diagnosis of BD I were selected by simple random probability sampling from the patients with bipolar disorder who came for follow-up and to receive their medications from psychiatric outpatient clinic. Results Themeanageoftheparticipantsrangedfrom18to40years,and51%werefemale. Patients with bipolar disorder with adult ADHD had more disability in work, social life, and family life than patients with pure bipolar disorder. Moreover, patients with bipolar disorder without adult ADHD had significant better quality of life in all domains as reflected by higher scores in WHO quality-of-life scale than patients with adult ADHD. Conclusion Adult ADHD is a frequent comorbid disorder in patients with BD. Adult ADHD comorbidity negatively affects functioning and quality of life of patients with BD. Keywords: Attention-deficit hyperactivity disorder, bipolar disorder, disability and quality of life