Background: Conjugated linoleic acid (CLA) is a supplemental dietary fatty acid that decreases fat mass accretion in young animals. Objective: The aim of this study was to determine CLA's efficacy with regard to change in fat and body mass index (BMI; in kg/m2) in children. Design: We conducted a 7 ± 0.5-mo randomized, double-blind, placebo-controlled trial of CLA in 62 prepubertal children aged 6-10 y who were overweight or obese but otherwise healthy. The subjects were randomly assigned to receive 3 g/d of 80% CLA (50:50 cis-9,trans-11 and trans-10,cis-12 isomers) or placebo in chocolate milk. Results: Fifty-three subjects completed the trial (n = 28 in the CLA group, n = 25 in the placebo group). CLA attenuated the increase in BMI (0.5 ± 0.8) compared with placebo (1.1 ± 1.1) (P = 0.05). The percentage change in body fat measured by dual-energy X-ray absorptiometry was smaller (P = 0.001) in the CLA group (-0.5 ± 2.1%) than in the placebo group (1.3±1.8%). The change in abdominal body fat as a percentage of total body weight was smaller (P = 0.02) in the CLA group (-0.09 ± 0.9%) than in the placebo group (0.43 ± 0.6%). There were no significant changes in plasma glucose, insulin, or LDL cholesterol between groups. Plasma HDL cholesterol decreased significantly more (P = 0.05) in the CLA group (-5.1±7.3 mg/dL) than in the placebo group (-0.7 ± 8 mg/dL). Bone mineral accretion was lower (P = 0.04) in the CLA group (0.05±0.03 kg) than in the placebo group (0.07 ± 0.03 kg). Reported gastrointestinal symptoms did not differ significantly between groups. Conclusions: CLA supplementation for 7 ± 0.5 mo decreased body fatness in 6-10-y-old children who were overweight or obese but did not improve plasma lipids or glucose and decreased HDL more than in the placebo group. Long-term investigation of the safety and efficacy of CLA supplementation in children is recommended.