Comorbidity of bipolar disorder (BD) with attention deficit hyperactivity disorder (ADHD) is frequent. The management of comorbid ADHD and BD is complicated by the risk of induction of (hypo) mania by the medications used for ADHD treatment. Earlier reports in children and adolescents with ADHD-BD suggest that the possibility of (hypo) mania induction is low when atomoxetine is used along mood stabilizers or antipsychotics. Here, we report induction of hypomania by atomoxetine when used for the treatment of comorbid ADHD in a BD patient while on prophylactic treatment with mood stabilizers. This report indicates that atomoxetine carries the risk of induction of (hypo) mania even in stabilized BD patients. Clinicians should closely monitor such patients for (hypo) mania symptoms.