Kaposi’s sarcoma is a systemic tumor multifocal disease of vascular genesis. In the classic Kaposi’s sarcoma, lesions are most often localized on the skin of the lower extremities, lesions of the mucous membranes are extremely rare. The article presents a rare clinical case of the classic Kaposi’s sarcoma in a 56 years old man with lesions of the oral mucosa. This clinical case is of particular interest due to the difficulties of differential diagnosis of Kaposi’s sarcoma and lichen planus when mucous membranes are involved in the pathological process. The presented case emphasizes the importance of a histological examination of the biopsy in differential diagnosis of Kaposi’s sarcoma. In case when the rash is widespread and the mucous membranes are involved, it is recommended to collect the material from the skin, because the histological picture will be more characteristic. It is important to pay attention to the duration of the elements’ existence, as morphological diagnosis in the early stages of Kaposi’s sarcoma can imitate the pattern of chronic dermatosis.