Our own lethal case of AIDS and the results of the study of other generalized viral infections, and literature data allow us to conclude that lymphoid system damage is the most characteristic feature of the disease. Hyperplasia of lymph nodes and the spleen occurs in the acute stage of the disease. Hyperplasia with subsequent desquamation of the sinus endothelium and alteration in dendritic reticular cells are found histologically. There is an accumulation in the sinuses of large cells with an abundant eosinophilic PAS-positive cytoplasm and a relatively clear nucleus, sometimes several nuclei are observed. Similar, in principle, changes as a manifestation of generalization, may occur in other organs, including alveolocytes and nephrothelium. All these changes suggest AIDS. The presence of immunodeficiency is confirmed by the unusually severe course of other infections, most frequently pneumocystosis.