To date, the modern Russian literature has not covered morphological studies of aortic aneurysm in Erdheim’s idiopathic cystic medial necrosis on autopsy and surgical materials, by using immunohistochemical studies, and it has not estimated the magnitude of pathohistochemical changes in the aortic media either. Objective: to conduct a morphological examination of aortic aneurysm in Erdheim’s idiopathic cystic medial necrosis, by using the material of pathology and forensic medical departments. Material and methods. 41 surgical samples from patients with idiopathic cystic medial necrosis of the ascending aorta and autopsy samples from 17 sudden deaths from aortic rupture were examined. The aortic wall was histologically and histochemically studied. Immunohistochemical examination was done using antibodies to smooth muscle actin, collagens types I and III, elastin, CD3 (T lymphocytes), CD20 (B lymphocytes), CD45 (leukocyte common antigen), CD68 (macrophages), apoptotic marker p53, tumor necrosis factor-α (TNF-α), and intercellular adhesion molecule-1 (ICAM-1). Results. Morphological examination showed lamellar unit destruction, fibrosis, medial necrosis, and elastic fiber fragmentation. These signs were assessed by their degree. The immunohistochemical examination of collagens types I and III determined fascicles of randomly positioned fibers. The multidirectional orientation of smooth muscle cells was confirmed by the expression of smooth muscle actin. There was an obvious expression of the apoptotic marker p53 in the smooth muscle cells of the aortic media and in the aortic adventitia. The expression of TNF-α was revealed in the cells of an inflammatory infiltrate in the aneurysm wall and that of ICAM-1 was found in the aortic endothelium, vasa vasorum, and in the cells of an inflammatory infiltrate in the media and adventitia. The paper proposes a design of postmortem and forensic medical diagnosis, which reflects the components of pathogenesis and tanatogenesis in Erdheim’s idiopathic cystic medial necrosis and which allows ICD-10 diagnosis coding. Conclusion. The main histopathological signs of aortic aneurysm in Erdheim’s idiopathic cystic medial necrosis were presented; structural changes in the primary components of the aortic wall were characterized; the cells of an inflammatory infiltrate were investigated; and the formulation of autopsy and forensic medical diagnoses was proposed.