Kaposi sarcoma is a malignant neoplasm most often developing on the basis of HIV infection. It is considered one of the indicator diseases of AIDS. Its diagnostics can be challenging in patients without other symptoms indicating immunodeficiency. A 35-year-old patient was admitted to the Clinic with an advanced tumor of the oral cavity after a period of two-year observation. One year earlier he underwent an excisional biopsy, however the result of the histopathological examination did not raise any suspicions of neoplastic growth. At the time of admission, he suffered from severe dehydration and malnutrition, reported pain when consuming fluids and food. Kaposi’s sarcoma was suspected and the diagnosis of HIV was confirmed. CT of the central nervous system showed signs of toxoplasmosis. Early diagnosis of HIV and efficient cooperation between various medical professionals can lead to an improved diagnostics of HIV and indicator diseases of AIDS, and therefore better treatment outcomes.