Acute pneumonias comprise a group of infectious diseases of different etiology which determines many features of clinico-anatomic manifestations of some of their forms including the extension of inflammatory lesions in the lungs. Usually developing due to disorders in the draining function of the bronchi, disorders of the phagocytic activity of leukocytes and alveolar macrophages as well as the presence of immunodeficient conditions, acute pneumonias emerge as complications of other diseases. From 1962 to the present time the rate of their detection in fatal cases increased from 11.6% to 42.8%. Their most frequent causative agents are staphylococci resistant to most antibiotics used for treatment, less frequently Pseudomonas aeruginosa and pathogenic fungi. Also, an increased role in the etiology of pneumonias of conditionally-pathogenic flora (Proteus, Acinetobacter, Enterobacter, Serratia) and frequent pneumonias caused by mixed microflora are observed. Pneumonias were the immediate cause of death in 19.2% of the fatal cases.