Clinical characteristics and course of disease of 19 pneumococcal, 11 adenoviral, 15 mycoplasmal and 10 mixed pneumonias, diagnosed in 55 military conscripts, were compared. Controls consisted of 104 conscripts with upper respiratory infections (URI). The triad: productive cough, blood stained sputum, and chest pain aggravated by breathing (pneumococcal score) distinguished pneumococcal and mixed pneumonias but not adenoviral and mycoplasmal pneumonias from URI. Higher C-reactive protein (CRP) and white blood cell (WBC) count distinguished the pneumococcal pneumonias, but not the other pneumonias, from URI. The pneumococcal scores and simple laboratory tests on admission were compared. The score effectively separated pneumococcal from adenoviral and mycoplasmal pneumonias, and patients with mixed infections from mycoplasmal infections. Higher CRP values and WBC counts distinguished pneumococcal pneumonia from other pneumonias. Auscultation revealed crackles in 27% of adenoviral and in 60-70% of mycoplasmal, pneumococcal and mixed pneumonias. Maxillary sinusitis was more common in pneumococcal (56%) than in mycoplasmal (7%) or mixed pneumonia (10%) or URI (14%). Pneumococcal pneumonias differed in most respects from the other groups. It is difficult to distinguish between adenoviral, mycoplasmal and mixed pneumonia and also URI.