The authors describe the case of a 7-year-old girl diagnosed with attention deficit hyperactivity disorder (ADHD) who, following an unsuccessful trial of stimulant medication and subsequent mood symptoms, was diagnosed with bipolar disorder. Following a comprehensive, multidisciplinary assessment, and withdrawal of her complex medication regimen, she was rediagnosed with ADHD. She displayed a positive response to behavioral parent training and pharmacological treatment with a long-acting stimulant. The case illustrates the benefits of a comprehensive, multidisciplinary evaluation and multimodal treatment. Her dramatic response to the long-acting stimulant suggests that many of her affective symptoms were due to stimulant "rebound" versus bipolar disorder. This case highlights the complexities of differentiating severe ADHD from bipolar disorder and suggests that stimulant rebound and other iatrogenic effects should be considered during the differential diagnostic process as potential mimics of bipolar disorder.