In addition to the assessment of the arterial lumen and coronary stenosis, endocoronary ultrasonography also provides information in vivo on the nature of atheromatous plaque. This study reports the preliminary correlations between the echographic appearances, the functional status of the patients and histological data of tissue obtained by directional atherectomy. The appearances of atheromatous infiltrations help distinguish stable forms of coronary disease with "hard", hyperreflective plaques, from acute forms with "soft" hyporeflective plaques. Ultrasonography confirms the clinico-pathological relationship of myocardial infarction and unstable angina with, in the acute forms, a similarity of the culprit plaques which appear vulnerable, hyporeflective and often complicated by the association of fissures and thrombus. The correlations with histological data are still incomplete. However, the "soft" nature of the plaque is an expression of its cellularity and lipid content whereas "hard" plaques correspond to increased fibrous tissue. These associations have no correlative values as yet. Nevertheless, they establish a parallelism which may eventually explain the functional variability and evolutivity of coronary artery disease. In the same way that visualisation of coronary stenosis led to the development of coronary artery surgery and later to angioplasty, observation of the plaque by ultrasonography should promote the development of pharmacological agents to treat the atheromatous lesions.