The goal of this study is to determine why some obese individuals develop insulin resistance and others do not. We hypothesize that an impairment in differentiation of fat cells (adipocytes) is responsible for the development of insulin resistance in select obese individuals. This study will evaluate obese individuals at baseline with respect to characteristics of adipocytes, including gene expression, and will then entail randomizing subjects to either weight loss or treatment with an insulin sensitizing drug (pioglitazone). Changes in insulin resistance will be associated with changes in adipocyte morphology and gene expression.