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Evidence is accumulating that influenza virus plays a role in the etiology of certain pneumonias generally considered to be primarily bacterial in origin. The earliest recorded observations (1, 2) describe influenza as a mild self-limited disease of the upper respiratory tract which progressed occasionally in certain individuals to produce more serious symptoms involving the pulmonary tissue. Soon after the discovery of bacteria, clinicians (3) pointed out that pneumonia caused by streptococci, staphylococci and pneumococci often followed symptoms of influenza.During the pandemic of 1918 influenza was frequently complicated by bacterial pneumonia. Certain investigators (4, 5) believed that the primary disease was of unknown virus etiology and that the pulmonary involvement was a manifestation of the secondary invasion of the respiratory tissues by pathogenic organisms.Epidemiological data (6-10) show a significant increase in pneumonia deaths accompanying and following epidemics of clinical influenza. Other similar data (11,12) indicate that localized epidemics of pneumococcus pneumonia have been associated with outbreaks of clinical influenza.Laboratory evidence (13-22) on clinical cases of bacterial pneumonia in man and experiments on laboratory animals (23-28) during the past decade have further supported these observations.The studies reported herein show that there was a simultaneous infection with pneumococci and influenza virus in about one-half of the human cases of lobar pneumonia studied during an influenza epidemic. Such mixed infections were also observed during interepidemic periods.