The effects of conjugated linoleic acid (CLA) on body weight and body composition in man are controversial. The aim of this study was to investigate the effects of milk supplementation with CLA on body composition and on the biochemical parameters of the metabolic syndrome. This was a randomised, double-blind, placebo-controlled trial. Subjects were randomised to a daily intake of 500 ml milk supplemented with 3 g CLA (using a mixture of the bioactive isomers cis-9, trans-11 and trans-10, cis-12, marketed as Tonalin, Naturlinea; Central Lechera Asturiana) or placebo for 12 weeks. Sixty healthy men and women (aged 35-65 years) with signs of the metabolic syndrome participated (BMI 25-35 kg/m2). Dual-energy X-ray absorptiometry was used to measure body composition (week 0 baseline and week 12). Total fat mass in the CLA-milk subgroup with a BMI < or = 30 kg/m2 decreased significantly while no changes were detected in the placebo group (approximately 2 %, P = 0.01). Trunk fat mass showed a trend towards reduction (approximately 3 %, P = 0.05). CLA supplementation had no significant effect on the parameters of the metabolic syndrome, nor was it associated with changes in haematological parameters or renal function. The supplementation of milk with 3 g CLA over 12 weeks results in a significant reduction of fat mass in overweight but not in obese subjects. CLA supplementation was not associated with any adverse effects or biological changes.