The usefulness of conjugated linoleic acid (CLA) as a nutraceutical remains ambiguous. Our objective was, therefore, to investigate the effect of CLA on body composition, blood lipids, and safety biomarkers in overweight, hyperlipidemic men. A double-blinded, 3-phase crossover trial was conducted in overweight (BMI ≥ 25 kg/m(2)), borderline hypercholesterolemic [LDL-cholesterol (C) ≥ 2.5 mmol/L] men aged 18-60 y. During three 8-wk phases, each separated by a 4-wk washout period, 27 participants consumed under supervision in random order 3.5 g/d of safflower oil (control), a 50:50 mixture of trans 10, cis 12 and cis 9, trans 11 (c9, t11) CLA:Clarinol G-80, and c9, t11 isomer:c9, t11 CLA. At baseline and endpoint of each phase, body weight, body fat mass, and lean body mass were measured by DXA. Blood lipid profiles and safety biomarkers, including insulin sensitivity, blood concentrations of adiponectin, and inflammatory (high sensitive-C-reactive protein, TNFα, and IL-6) and oxidative (oxidized-LDL) molecules, were measured. The effect of CLA consumption on fatty acid oxidation was also assessed. Compared with the control treatment, the CLA treatments did not affect changes in body weight, body composition, or blood lipids. In addition, CLA did not affect the β-oxidation rate of fatty acids or induce significant alterations in the safety markers tested. In conclusion, although no detrimental effects were caused by supplementation, these results do not confirm a role for CLA in either body weight or blood lipid regulation in humans.