Among 72 adult patients with a diagnosis of acute bronchitis, serological investigation established the presence of an aetiologic agent in 29 (40%). Influenza virus was the most common pathogen. Seven patients had bacterial infection, caused by pneumococci in four patients and Mycoplasma pneumoniae in three. Five of the patients had pneumonia as diagnosed by radiography, and mycoplasmal aetiology was established in one of these. Altogether, 11 patients either had bacterial infection or radiographic pneumonia. Although the doctors' recording of wheezes was strongly associated with prescription of antibiotics (p < 0.0001), wheezes were heard only in two of the 11 patients with pneumonia or bacterial infection, compared with 30 of the 61 patients with viral or unspecified bronchitis. The median value of C-reactive protein (CRP) was 52 mg/l in the 11 patients, significantly higher than < 11 mg/l in the 61 other patients (p < 0.0001). The corresponding values for erythrocyte sedimentation rate were 45 and 14 mm/h (p < 0.0005). The results indicate that certain patients with acute bronchitis should be treated with antibiotics, and that the erythrocyte sedimentation rate and the CRP-test may be useful in detecting which patients this applies to.