In this study, we examined whether in vivo keishi-bukuryo-gan (a Kampo formulation) could prevent the progression of atherosclerosis in cholesterol-fed rabbits, an animal model for hypercholesterolaemia. Sixteen male Japanese white rabbits (2 kg body weight) were divided into two groups. Group A (n = 8) was fed standard rabbit chow containing 1% cholesterol for 8 weeks. Group B (n = 8) was fed standard rabbit chow containing 1% cholesterol and 1% keishi-bukuryo-gan for 8 weeks. At the end of the experiment, average plasma concentrations of total-cholesterol and IDL-cholesterol were 2055.9 AE 201.8 mg/dL and 408.1 AE 62.6 mg/dL in group A and 1950.5 AE 126.3 mg/dL and 407.6 AE 56.6 mg/dL in group B, respectively. The percentage of the surface area of the total thoracic aorta with visible plaque was significantly reduced by keishi-bukuryo-gan administration; group A was 33.2% AE 5.3% and group B was 14.3% AE 2.9%. b -very low density lipoprotein (VLDL) and low density lipoprotein (LDL) isolated from cholesterol fed rabbits treated with keishi-bukuryo-gan (group B) were shown to be highly resistant to oxidative modification by cupric ion. Sera isolated from rabbits administered keishi-bukuryo-gan had reduced lipid peroxide formation compared with those from rabbits without keishi-bukuryo-gan. Thus, keishi-bukuryo-gan prevents the progression of atherosclerosis in cholesterol-fed rabbits in vivo by limiting oxidative LDL modification.