Streptococcus pneumoniae is seldom considered as an etiologic agent of necrotizing or cavitating pneumonia. However, during a 5-month period we encountered four patients, bacteremic with S. pneumoniae, with such a pulmonary process. Review of the older literature indicates that this association may be more frequent than is commonly assumed. Anatomic, physiologic, and immunologic alterations of the pulmonary defense mechanisms prior to and during the infection as well as virulence factors of S. pneumoniae (i.e., rapid multiplication, accumulation of capsular polysaccharides, and inhibition of phagocytosis) in concert may produce the resultant decrease in bacterial clearance from the lung with the consequent necrosis of lung parenchyma. Since sputum and blood cultures are reported to be positive in only 50 percent and 25 percent, respectively, of cases of pneumonia, etiologic diagnosis may be difficult. Nevertheless, S. pneumoniae must be considered in the differential diagnosis of the patient with necrotizing or cavitating pneumonia.