To assess the early stage of atherosclerosis of the thoracic descending aorta, we evaluated morphological atheromatous lesions (atherosis) and the stiffness parameter of the artery (β; sclerosis) in 24 male rabbits using echography. Male Japanese white rabbits weighing 2.5-3.0 kg were fed a diet containing 1% cholesterol for 7 (n=8) or 14 weeks (n=8). Rabbits fed a normal diet were used as controls (n=8). Atheromatous lesions were evaluated with intravascular ultrasound (IVUS: Aloka, 20 MHz, 6F). We also calculated β using M-mode echography (7.5 or 10 MHz) and direct aortic pressure measurement. Thickening of the intima-media complex was clearly observed with IVUS in the 14-week group but was not detected in the others. Histologically, only a thin layer of foamy cells on the intima (thickness <20 μm) was observed in the 7-week group. The value of β was significantly increased in both the 7-week (4.7±2.2) and 14-week groups (4.5±0.8) compared with controls (1.7±0.9, both p<0.01). These results suggest that the development of atherosis might be preceded by vascular sclerosis during the early stage of atherosclerosis when the serum cholesterol level is high: at a time when the thin layer of foamy cells could not be detected by conventional IVUS. (Jpn Circ J 1996; 60: 691 - 698)