Uncoupling protein 2 (UCP2) is a possible target molecule for energy dissipation. Many dietary fats, including safflower oil and lard, induce obesity in C57BL/6 mice, whereas fish oil does not. Fish oil increases UCP2 expression in hepatocytes and may enhance UCP2 activity by activating the UCP2 molecule or altering the lipid bilayer environment. To examine the role of liver UCP2 in obesity, we created transgenic mice that overexpressed human UCP2 in hepatocytes and examined whether UCP2-transgenic mice showed less obesity when fed a high-fat diet (safflower oil or lard). In addition, we examined whether fish oil had anti-obesity effects in UCP2-knockout mice. UCP2-transgenic and wild-type mice fed a high-fat diet (safflower oil or lard) developed obesity to a similar degree. UCP2-knockout and wild-type mice fed fish oil had lower rates of obesity than mice fed safflower oil. Remarkably, safflower oil did not induce obesity in female UCP2-knockout mice, an unexpected phenotype for which we presently have no explanation. However, this unexpected effect was not observed in male UCP2-knockout mice or in UCP2-knockout mice fed a high-lard diet. These data indicate that liver UCP2 is not essential for fish oil-induced decreases in body fat.