Abstract —The plasma cholesteryl ester transfer protein (CETP) plays a major role in the catabolism of HDL cholesteryl ester (CE). CETP transgenic mice have decreased HDL cholesterol levels and have been reported to have either increased or decreased early atherosclerotic lesions. To evaluate the impact of CETP expression on more advanced forms of atherosclerosis, we have cross-bred the human CETP transgene into the apoE knock-out (apoE0) background with and without concomitant expression of the human apo A-I transgene. In this model the CETP transgene is induced to produce plasma CETP levels 5 to 10 times normal human levels. CETP expression resulted in moderately reduced HDL cholesterol (34%) in apoE0 mice and markedly reduced HDL cholesterol (76%) in apoE0/apoA1 transgenic mice. After injection of radiolabeled HDL CE, the CETP transgene significantly delayed the clearance of CE radioactivity from plasma in apoE0 mice, but accelerated the clearance in apoE0/apoA1 transgenic mice. ApoE0/CETP mice displayed an increase in mean atherosclerotic lesion area on the chow diet (approximately 2-fold after 2 to 4 months, and 1.4- to 1.6-fold after 7 months) compared with apoE0 mice ( P <0.02). At 7 months apoA1 transgene expression resulted in a 3-fold reduction in mean lesion area in apoE0 mice ( P <0.001). In the apoE0/apoA1 background, CETP produced an insignificant 1.3- to 1.7-fold increase in lesion area. In further studies the CETP transgene was bred onto the LDL receptor knock-out background (LDLR0). After 3 months on the Western diet, the mean lesion area was increased 1.8-fold ( P <0.01) in LDLR0/CETP mice, compared with LDLR0 mice. These studies indicate that CETP expression leads to a moderate increase in atherosclerosis in apoE0 and LDLR0 mice, and suggest a proatherogenic effect of CETP activity in metabolic settings in which clearance of remnants or LDL is severely impaired. However, apoA1 overexpression has more dramatic protective effects on atherosclerosis in apoE0 mice, which are not significantly reversed by concomitant expression of CETP.