The objective of this study was to clarify whether morphologic evaluation of the in vivo artery with intravascular ultrasound provides as sensitive a marker as endothelial dysfunction or microscopic histologic assessment. Endothelial dysfunction assessed with the changes in the vessel diameter during acetylcholine infusion has been used as a more sensitive marker of atherosclerosis than the angiographic estimates of morphologic structure of the vessel. Recent advent of intravascular ultrasound has provided such high-resolution images of the vessels that morphologic changes in the vessel structure are sensitively and accurately detected. Twenty-two rabbits were divided into three groups: six rabbits fed a cholesterol-rich diet for 2 weeks as the hypercholesterolemia group, eight rabbits fed with the diet for 8 weeks as the atherosclerosis group, and eight rabbits fed a normal diet as the normal group. After evaluating the atherosclerotic lesions by intravascular ultrasound, the cross-sectional area was measured in the baseline and during the infusion of acetylcholine (0.05, 0.5, and 5 micrograms/kg/min) and nitroglycerin (5 micrograms/kg/min). No atherosclerotic lesions were detectable with intravascular ultrasound in any rabbit despite the presence of microscopic intimal lesions in the vessels in the rabbits of the atherosclerosis group. The cross-sectional area increased during acetylcholine infusion in the rabbits of the normal and the hypercholesterolemia groups. In contrast, in the rabbits of the atherosclerosis group, the cross-sectional area did not significantly increase during acetylcholine infusion at the rate of 0.5 microgram/kg/min and even tended to decrease at the rate of 5 micrograms/kg/min (-3.8% +/- 3.7%, P < 0.05 vs the normal group). Dilating responses to nitroglycerin infusion were similar among all three groups. In conclusion, impairment of the endothelium-dependent vasodilating response assessed with intravascular ultrasound in the in vivo vessel precedes the appearance of echographic atherosclerotic findings. Thus intravascular ultrasound, if used in combination with drug intervention to assess endothelial function, would provide even more accurate assessment of the vessels than conventional intravascular ultrasound alone.