This study was designed to investigate the efficacy and tolerability of daily conjugated linoleic acid (CLA) in healthy exercising humans. This was a random-double-blind, placebo-controlled study in 20 healthy humans of normal body weight and body mass index less than 25.0 kg/m2, who did standardized physical exercise in a gym for 90 min three times weekly. Participants took either placebo (hydrogel) or CLA 0.6 g [DOSAGE ERROR CORRECTED], three times daily, as two capsules during meals, for 12 weeks. Body fat, measured using near infrared light, was significantly reduced in the CLA group during the study, but not in the placebo group. No effects on body weight were observed. Tolerability was good and similar in the two groups. Compliance, as judged by the number of returned capsules, was more than 80% of the recommended dose for all participants. Thus CLA reduces body fat but not body weight in healthy exercising humans of normal body weight.