Despite the fact that mutations resulting in the absence of leptin or its receptor have been associated with severe obesity and diabetes, such mutations do not appear to be responsible for most human obesity. Indeed, diet-induced obesity in animals and humans has been characterized by hyperleptinemia. This has been interpreted as evidence for leptin resistance. However, no careful longitudinal studies evaluating the role of leptin in the development of obesity exist. We report a series of studies in A/J and C57BL/6J (B/6) mice that demonstrate a direct relationship between the ability to increase plasma leptin levels in response to a high-fat diet and resistance to the subsequent development of obesity and diabetes. While leptin levels are similar in lean, low-fat-fed A/J and B/6 mice, the effects of a high-fat diet on plasma leptin differ dramatically between the two strains. After 4 weeks of high-fat feeding, leptin levels in A/J mice increased 10-fold, and this elevated level was maintained independent of weight gain throughout a 14-week feeding period. However, in B/6 mice, leptin levels remained at least twofold lower and only rose very gradually along with a significant increase in adiposity, hyperglycemia, and hyperinsulinemia. These differences in the response of leptin to diet are independent of food intake and plasma insulin levels during the 1st month of feeding. Further, we demonstrated that leptin administration did not influence the expression of the novel uncoupling protein UCP2, which also responds to dietary fat. From these results, we suggest that the response of leptin to fat feeding may be an important predictor of the development of subsequent obesity.