Conjugated linoleic acid (CLA) comprises a group of unsaturated fatty acid isomers with a variety of biological effects. CLA reduces body fat accumulation in animal models and has been ascribed significant effects on lipid and glucose metabolism. It has been suggested that the trans-10,cis-12 isomer is the active isomer with regard to antiobesity and insulin-sensitizing properties. The metabolic effects in humans are not well characterized. We have investigated and published the effects of CLA (given as the commercially available mixture and as the purified trans-10,cis-12 isomer) on anthropometry, lipid and glucose metabolism, and markers of lipid peroxidation. The results from those studies indicate that CLA might slightly decrease body fat in humans, particularly abdominal fat, but there is no effect on body weight or body mass index. There is no simultaneous improvement in lipid or glucose metabolism. Rather, the trans-10,cis-12 CLA isomer unexpectedly caused significant impairment of the peripheral insulin sensitivity as well as of blood glucose and serum lipid concentrations. In addition, CLA markedly elevated lipid peroxidation. Thus, the metabolic effects of CLA in humans seem complex, and further studies, especially of specific isomers and of longer duration, are needed.