A study is presented of 582 patients with acute viral-bacterial pneumonia in those with a history of influenza and acute respiratory disease (ARD). Protracted course of the disease was observed in 121 (20.8%) and 461 (79.2%) the course of pneumonia was acute. It is shown that the formation of protracted of acute pneumonia in patients with influenza and ARD is furthered by several factors: age, foci of chronic infection, a history of inflammation, increased level of circulating immune complexes, late hospitalization and inadequate therapy. Experiments on Syrian hamsters with induced parainfluenzal infection showed that mixed viral-bacterial infection is more severe than monoinfection.