Background: This study aims to establish if adult patients with major depressive disorder (MDD) and childhood Attention Deficit/Hyperactivity disorder (ADHD) symptoms would be more frequently within the bipolar spectrum than depressed patients without childhood ADHD. Methods: This study was carried out in outpatients recruited by psychiatrists in private practice, with 3963 participants being included in the final sample. Clinicians filled out questionnaires about current depressive symptoms in their patients, lifetime bipolar symptoms, global assessment of functioning and parental history of both major depression and bipolar disorder. Patients assessed current level of anxiety and depressive symptoms and antecedents of childhood ADHD symptoms. Results: Depressed adults with significant childhood ADHD symptoms had a specific pattern of their major depressive episode compared to depressed patients without such symptoms. Subjects with childhood ADHD symptoms were more likely to report lifetime symptoms of mania/hypomania and to have a parent with type I or II bipolar disorder. The developmental trajectories of familial risk for lifetime bipolar symptoms showed that parental bipolar disorder influenced lifetime bipolar symptoms both through a direct pathway and an indirect pathway involving childhood ADHD symptoms. Childhood ADHD and number of depressive symptoms both made direct contributions to lifetime bipolar symptoms.