Several prominent health benefits have been attributed to consumption of conjugated linoleic acid (CLA). In rodents, dietary CLA stops cancer growth (1), improves glucose tolerance (2), reduces body fat, and increases oxygen consumption (3) (4) (5). The mechanism by which CLA mediates any of these benefits is yet to be determined. However, Ip et al. (1) observed that an inhibitory effect of CLA on mammary epithelial cell growth was associated with apoptosis. This clue, coupled with the observation that centrally administered leptin can cause apoptosis of adipocytes (6), led us to test the hypothesis that dietary CLA causes apoptosis of adipocytes. We also determined effects of dietary CLA in lines of mice that differed genetically with regard to obesity.