CLA has a range of biological properties, including effects on lipid metabolism and body composition in experimental animals. The prevalent isomer of CLA found in the human diet is 9c,11 t-CLA, and it is predominantly found in products containing fat from ruminant animals. This study investigated the effect of dietary CLA on energy balance in mice. Synthetic CLA reduced body fat in growing male BALB/c mice in a dose-dependent manner over the range 0.25-1.0% w/w CLA in the diet. Weight gain was also reduced at the highest levels of dietary CLA, being only 5.88 +/- 2.68 g/4 mice (mean +/- 1 SD) after 4 wk of 2.0% CLA in the diet, compared with weight gains of 7.51 +/- 2.22 to 8.17 +/- 2.34 g/4 mice in the 0-0.5% CLA groups. There was no significant effect on weight gain if diets contained 0.5% synthetic CLA or less. These results suggest that high levels of a synthetic mixture of CLA isomers modify energy metabolism and body composition and that high levels of synthetic CLA impair weight gain and reduce body fat pad mass in growing mice.