Because being overweight increases the risk for type 2 diabetes, hypertension, and coronary heart disease, the rapid increase in the prevalence of overweight and obesity in the United States represents a major health problem. The relationship between overweight and obesity and these conditions is probably due to insulin resistance and compensatory hyperinsulinemia. However, although it is known that weight loss in insulin-resistant and hyperinsulinemic persons will be of substantial metabolic benefit, it is equally well established that many overweight and obese persons are not insulin resistant. In the absence of insulin resistance and its manifestations, the risk for type 2 diabetes, hypertension, and coronary heart disease is reduced and the metabolic benefit of weight loss in the substantial number of overweight persons who are insulin sensitive is relatively minimal. Consequently, it is important to identify which overweight persons are most likely to be insulin resistant by considering their family history; blood pressure; and plasma glucose, triglyceride, and high-density lipoprotein cholesterol concentrations. Thoughtful use of this information will help identify the subset of persons who will benefit the most from intense therapeutic efforts to lose weight.