We have previously demonstrated that the C57BL/6J (B 6J ) mouse will develop severe obesity, hyperglycemia, and hyperinsulinemia if weaned onto a high-fat, high-sucrose (HH) diet. In the present study, we compared the effects of fat and sucrose separately and in combination on diabetes- and obesity-prone B 6J and diabetes- and obesity-resistant A J mice. After 4 months, the feed efficiency ([FE] weight gained divided by calories consumed) did not differ across diets in A J mice, but B 6J mice showed a significantly increased FE for fat. That is, B 6J mice gained more weight on high-fat diets without consuming more calories than A J mice. The increase in FE was related to adipocyte hyperplasia in B 6J mice on high-fat diets. Fat-induced obesity in B 6J mice was unrelated to adrenal cortical activity. In the absence of fat, sucrose produced a decreased in FE in both strains. Animals fed a low-fat, high-sucrose (LH) diet were actually leaner than animals fed a high—complex-carbohydrate diet. Fat was also found to be the critical stimulus for hyperglycemia and hyperinsulinemia in B 6J mice. In the absence of fat, sucrose had no effect on plasma glucose or insulin. These data clearly show that across these two strains of mice, genetic differences in the metabolic response to fat are more important in the development of obesity and diabetes than the increased caloric content of a high-fat diet.