Data, including clinical features of bronchopulmonary infections, i.e. cough with purulent sputum, fever, peripheral blood leucocytosis, patchy pulmonary shadows on chest radiographs and the presence of crepitations or dullness on examination of the chest, were prospectively recorded in 107 consecutive adult patients whose expectorated sputum yielded Branhamella-like bacterial colonies in routine cultures. Subsequently, isolates from 26 patients were confirmed to be Branhamella catarrhalis. Of 81 patients (unmatched controls) with confirmed commensal Neisseria species, 40 (matched controls) were age and sex matched with the 26 patients (cases) positive for B. catarrhalis. Clinical features of bronchopulmonary infection were significantly more among cases compared to unmatched controls. Compared with matched controls, significantly more of the cases had three or more clinical features of bronchopulmonary infection (P less than 0.05). This is the first study of the clinical significance of B. catarrhalis in expectorated sputum that included controls to circumvent the confounding effects of oropharyngeal and airway colonisation in patients with bronchopulmonary infections. The isolation of this organism in routine sputum culture was found to be significantly related to clinical infection.