The sonographical, morphological, and immunohistochemical Chlamydia pneumoniae characteristics of the atherosclerotic plaques from 200 patients with asymptomatic (A =59) and symptomatic (S=141) courses of carotid stenosis (A/S-1/2.4) were studied. 34 (17 %) homogeneous (HO) or stable and 166 (83%) heterogeneous (HE) or unstable plaques were found morphologically. According to the number of their histopathological signs and prevalence of inflammatory cell type, the plaques were subdivided into 3 HO and 5 HE subtypes with different sonographical characteristics. Complicated plaques, in both A and S patients, have signs of the activation of chronic inflammation (PMN), of phagocytosis (foreign body macrophages), and of atheromatosis (foam cells) all together in HE plaque subtypes (HE2, HE5, HE4) and only of atheromatosis in HO plaques (HO2). Disruptures, intraplaque haemorrhages, and thrombi were associated with the C. pneumoniae, as the inflammatory agent, found in every kind of phagocytes located in the fibrous cap, atheroma, and mainly at the boundary between the fibrous cap and atheroma. The complications strongly correlate with the abundance of C. pneumoniae (p < 0.001) and so with a greater risk of a cerebral ischaemic stroke as in A and in S patients with carotid stenosis. Reliable correlation between C. pneumoniae and the degree of carotid stenosis and plaque thickness (p < 0.06 - 0.07) as well as sufficient correlation between plaque thickness and the disease’s symptomaticity (p < 0.01) were found
