The clinical signs, symptoms and host responses (erythrocyte sedimentation rate, white blood cell count and C-reactive protein) were studied to distinguish bacterial from viral acute lower respiratory infection (ALRI) in 121 children hospitalized for ALRI. Etiologic diagnosis was based on blood culture, antibody assays and antigen detection. Children with bacterial involvement only were older than those with viral involvement alone (mean, 5.1 vs. 2.5 years), and their duration of respiratory symptoms had lasted longer (mean, 4.6 vs. 3.3 days). Children with unknown etiology had a shorter duration of fever before hospitalization than those with etiology identified with the methods used (mean, 1.6 vs. 2.9 days). The host response ranged widely within etiologic groups. The mean erythrocyte sedimentation rate did not differ significantly between the bacterial and viral ALRI (38 vs. 28 mm/hour); neither did white blood cell count (13.2 vs. 13.6 x 10(9)/liter) or C-reactive protein (68 vs. 49 mg/liter). No combination of clinical signs and host responses or any cutoff values could be shown to differentiate reliably bacterial from viral ALRI.