We studied the effects of high cholesterol diet and atherosclerosis on the development of left ventricular hypertrophy (LVH) in rabbits. The suprarenal aorta was banded in rabbits on a normal diet and in rabbits on a 4% cholesterol diet. A group of sham-operated rabbits was fed a normal diet, and another group of sham-operated rabbits was fed a 4% cholesterol diet. All four groups were sacrificed after approximately 6 months, when hypertrophy in the banded rabbits had stabilized. The LV- and RV-to-body weight ratios were determined. LV to body weight increased from a value in controls of 0.78 ± 0.052 to 1.02 ± 0.085 gm/kgm (P < 0.05) in the banded group on a normal diet and to 1.03 ± 0.031 gm/kgm (P < 0.05) in the sham-operated group on a cholesterol diet. The increase in the LV-to-body weight ratio to 1.21 ± 0.072 gm/kgm was largest (P < 0.05) in the banded group on the high cholesterol diet. RV to body weight did not change in any group. The development of LVH in atherosclerotic rabbits was associated with an increase in arterial pressure. Myocardial ischemia could not be responsible for LVH since hypertrophy was confirmed only in the left ventricle even in the presence of right ventricular myocardial ischemia. Atherosclerosis increased the degree of LVH induced by aortic banding in an additive manner. Therefore, the findings indicate that LVH in the atherosclerotic rabbit was due to increased arterial pressure induced by the atherosclerosis and that atherosclerosis further increased the degree of hypertrophy induced by aortic banding by increasing the arterial pressure proximal to the band. © 1976.