Adult patients admitted to the medical service of a city-county hospital for treatment of presumptive acute pneumonia were followed up in a prospective study of pneumonia etiology from July 1, 1967, to June 30, 1968. Laboratory specimens were tested for bacterial, mycoplasmal, and viral agents, and results, supported by serologic data, were evaluated. Of 292 cases of pneumonia, 167 (57%) could be attributed to specific bacterial pathogens. The pneumococcus was the most frequent etiologic agent (62%), gram-negative bacilli the second most frequent (20%), and Staphylococcus aureus the third (10%). Viral infections other than influenza appeared to be unrelated to adult pneumonias. Four fifths of the patients had one or more associated diseases. Overall mortality was 24%. An age of 40 years and over and gram-negative bacillus or staphylococcal etiology were identified as mortality risk factors.