Despite a characteristic clinical picture, hemolytic streptococcal involvement of the lung rarely is considered in the differential diagnosis of pneumonia today. The availability of effective chemotherapeutic agents has led to a lack of awareness of the natural history. This report of 5 cases furnishes an opportunity to review the clinical picture, the pathogenesis, and the complications. Report of Cases Case 1.— A 19-year-old white male was hospitalized in March, 1955, and was too ill to give any history. Interrogation later revealed that for 2 weeks he had suffered from a "common cold." On the day before admission he suddenly developed shaking chills, fever, and severe right pleuritic pain. His previously dry cough became productive of thick, tenacious, yellow-green sputum. He was an acutely ill, disoriented, cyanotic white male. The chest was splinted, not dull to percussion, with moist rales at the right base, axilla, and left base. He had a