A prospective study of the etiologies and characteristic features of pneumonias was done at the Milwaukee County General Hospital from October 1969 to March 1970. The study comprised 148 consecutive patients who had received no previous antibiotics.D. pneumoniaewas the etiologic agent in 79 patients (53.0 per cent); 21.6 per cent were bacteremic. According to strict criteria, 9.5 per cent of pneumonias were ascribed to gram-negative bacilli and 7.0 per cent to staphylococci. Primary viral and mycoplasmal infections were diagnosed by complement fixation test in 15.6 per cent of the 77 pneumonia patients from whom paired sera were available. In 17.0 per cent, no etiology could be established.D. pneumoniaepatients with and without bacteremia differed from each other significantly (p<0.05) in mortality (35.0 per centvs13.0 per cent), days febrile before admission (4.1vs2.2), shock (23.0 per centvs4.8 per cent), and lobar pattern (88.2 per centvs52.0 per cent), respectively. In comparing pneumonias of different etiologies, patients with gram-negative bacillary pneumonias were found to be significantly older (70 yr) than those with other pneumonias (56 yr; p<0.01), had a higher mortality (47.0 per centvs13.8 per cent; p<0.01), and tended more often to come from nursing homes (59.0 per centvs28.0 per cent; p<0.05).