A number of experts recommend the use of insulin for patients with non-insulin-dependent diabetes mellitus (NIDDM) who fail to respond to diet, exercise, and oral hypoglycemics, even when the patient is morbidly obese. This article describes the use of insulin in two obese patients with NIDDM whose obesity worsened following the institution of insulin therapy. In some cases the risk for increased obesity and its complications following the institution of insulin may offset the potential benefits of insulin therapy itself. There are two main drawbacks associated with insulin therapy in these patients. First, from a medical point of view, insulin has a lipogenic effect and may actually contribute to weight gain, hyperinsulinemia, and increased insulin resistance in obese patients with NIDDM. Second, from a behavioral point of view, the institution of insulin therapy may shift the patient's and physician's focus from the preferred lifestyle adjustments to the numerous details associated with insulin use and monitoring. Since weight gain and sedentary activity are themselves risk factors for coronary artery disease, the benefits of decreased blood glucose levels should be balanced against the risk of increased weight gain in these patients.