Objective: To determine the effect of supplementation with a combination of conjugated linoleic acids (CLA) and n-3 long chain polyunsaturated fatty acids (LC-PUFA) on adiposity and insulin sensitivity in healthy lean and obese human volunteers. Research Methods and Procedures: Lean (BMI 23.6 ± 1.5kg/m2) (n=14) and obese (BMI 32.5 ± 1.8kg/m2) (n=13), young men (31.4 ± 3.9yrs) and lean (BMI 23.6 ± 1.4kg/m2) (n=22) and obese (BMI 31.9 ± 1.6kg/m2) (n=16) older men (56.6 ± 4.6yrs) participated in a double-blind placebo-controlled, randomised crossover study. Subjects were supplemented with either 6g/d placebo or 3g/d CLA (50:50 mixture of cis-9, trans-11 and trans-10, cis-12 isomers) and 3g/d n-3 LC-PUFA for 12-weeks with a 12-week wash out period between crossovers. Body composition was assessed by DXA before and after each supplementation period. Fasting plasma glucose, insulin, adiponectin and leptin concentrations were measured and insulin resistance estimated by the homeostasis model assessment for insulin resistance (HOMA-IR). Results: In the younger obese subjects only, compared with placebo, supplementation with CLA+n-3 LC-PUFA prevented the increase (11%) in abdominal fat mass, increased fat free mass (by 2.5%) and increased plasma adiponectin levels (by 9%). Fasting glucose levels were increased in the older obese subjects but there were no significant effects on HOMA-IR in any of the groups. Conclusions: The combination of 3g/d CLA with 3g/d n-3 LC-PUFA may prevent increased abdominal fat mass in younger (aged 20-37y) obese individuals without deleteriously effecting insulin sensitivity, whereas younger lean and older lean and obese individuals show no measurable differences.