Bacteriologic and virologic investigations carried out in a group of 85 patients with acute pneumonia have allowed the determination of the etiologic agent in 52.9% of the cases. Of these, 40% showed combined viral and bacterial infections. Mixovirus infections were frequently associated with staphylococcic ones (7 out of 8 cases) and adenovirus infections with the pneumococcic ones (6 out of 7 cases). The most severe and longdrawn clinical pictures with frequent pleuropulmonary complications (in 22.24% of the cases) had a biphasic evolution in only 6 of the 18 pneumonias of mixed etiology. For their high frequency and their clinical peculiarities, mixed etiology pneumonias might be considered as distinct clinical entities.