The background of AIDS is the deregulation of the cellular immune system caused by HIV infection. AIDS has proved to be entirely different from any previous known diseases. It affects practically all the tissues and organs of the human body. Clinical and autopsy diagnoses are confronted with an incomparable diversity of inflammatory, vascular, degenerative and neoplastic changes. Thanks to clinical and pathological researshes, the medical knowledge of AIDS has rapidly progressed. The Institute of Forensic Medicine is, for now, the unique medical institution organized to perform obligatory autopsies of drug-addicts, to detect the other cases suspect of HIV infection or some AIDS indicative diseases. The aim of this study is to establish the basis for multidisciplinary investigations of AIDS and to solve very important problems such as: a) to accept the diagnostic criteria for correct diagnosis of AIDS, with or without laboratory evidence of HIV infection; b) to recognize the early stadium of clinico-pathological entities associated with HIV infection; c) to improve the "indicator" of AIDS disease for daily medical practice.