The clinical features of Eaton agent pneumonia, adenovirus pneumonia, and a group of etiologically undiagnosed viral pneumonias were compared. Eaton agent pneumonia was found to be an acute febrile illness characterized by cough, fever, chills, headache, and malaise. The duration of illness in untreated patients averaged between 2 and 3 weeks. Clinical findings in the other viral pneumonias were milder with less systemic symptoms, less pulmonary involvement, and shorter duration. It was not possible to distinguish Eaton agent pneumonia from the other types on the basis of clinical findings alone, without recourse to the fluorescent antibody test. However, the development of cold agglutinins suggested a diagnosis of Eaton agent pneumonia since they appeared in 49 of 109 patients with this illness, but were infrequently found in other viral pneumonias.