Introduction: Obesity has emerged as a novel risk factor for impaired cognitive performance, independent of other comorbid medical conditions. Given that patients with Major Depressive Disorder (MDD) or Bipolar Disorder (BD) experience higher rates of obesity and metabolic syndrome than the general population, this metabolic derangement may play a role in the cognitive dysfunction seen in this patient population. A challenge in assessing this, however, is that most interventions do not result in a significant weight loss. This is not true of bariatric (weight loss) surgery, which results in weight loss in excess of 75 pounds in one year. The goal of this study is to examine the impact of weight loss on cognition in patients with MDD and BD, by examining cognitive performance pre- and post-bariatric surgery. Methods: This study will compare cognitive functioning in 20 obese individuals (BMI > 35kg/m2) with BD, 20 with MDD, and 20 obese and 20 non-obese controls prior to surgical intervention. Cognitive performance will be assessed one month prior to and one year post intervention, via a standardized battery of neuropsychological tests. An fMRI investigation allowing assessment of neuroanatomical change associated with weight change will also occur at each time point. Results: Interactions between obesity, mood disorders and cognition will be discussed. Conclusions: Obesity may have a negative impact on cognition that is exacerbated in the presence of a mood disorder. Given that different psychiatric drugs confer different risks of weight gain, this study will impact treatment in this vulnerable population.