Unfavorable blood lipid levels, such as elevated levels of total cholesterol, LDL cholesterol, and triglycerides, and reduced levels of HDL cholesterol, have been associated with increased risk of CVDs ( 1-3 ). Lipid levels are determined by genetic and environmental factors, as well as their interactions ( 4, 5 ). There has been great interest in investigating cholesteryl ester transfer protein (CETP), a plasma glycoprotein that facilitates the transfer of cholesteryl ester and triglycerides between HDL and other lipoproteins ( 6 ), in relation to blood lipids. CETP defi ciency due to CETP gene mutation causes increased HDL cholesterol levels ( 7,8 ). In patients with low Abstract Little is known about whether cholesteryl ester transfer protein ( CETP ) genetic variation may modify the effect of weight-loss diets varying in fat content on changes in lipid levels. We analyzed the interaction between the CETP variant rs3764261 and dietary interventions on changes in lipid levels among 732 overweight/obese adults from a 2 year randomized weight-loss trial [Preventing Overweight Using Novel Dietary Strategies (POUNDS LOST)], and replicated the fi ndings in 171 overweight/obese adults from an independent 2 year weight-loss trial [Dietary Intervention Randomized Controlled Trial (DIRECT)]. In the POUNDS LOST, participants with the CETP rs3764261 CC genotype on the high-fat diet had larger increases in HDL cholesterol ( P = 0.001) and decreases in triglycerides ( P = 0.007) than those on the low-fat diet at 6 months, while no signifi cant difference between these two diets was observed among participants carrying other genotypes. The gene-diet interactions on changes in HDL-cholesterol and tri glyc erides were replicated in the DIRECT (pooled P for interaction ≤ 0.01). Similar results on trajectory of changes in HDL cholesterol and triglycerides over the 2 year intervention were observed in both trials. Our study provides replicable evidence that individuals with the CETP rs3764261 CC genotype might derive greater effects on raising HDL cholesterol and lowering triglycerides by choosing a low-carbohydrate/high-fat weight-loss diet instead of a low-fat diet. -Qi, Q