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Objectives: To assess the pathogenic significance of Branhamella catarrhalisisolates in patients with respiratory infections and to define the clinical characteristics of such patients. Design and setting: Respiratory specimens were assessed in a three‐year prospective study performed in a Brisbane metropolitan hospital. Assessment' of the pathogenic significance of isolates of B. catarrhalis was based on four predetermined criteria: (i) clinical evidence of respiratory infection based on history, examination and chest x‐ray; (ii) isolation of B. catarrhalis as the sole potential pathogen; (iii) absence of antibiotic treatment in the previous two weeks; and (iv) subsequent clinical response to an antibiotic to which the isolate was sensitive. Results: B. catarrhalis was identified in 118 respiratory samples, 92 (78%) being from patients less than 10 years old. Infection with B. catarrhalis was more commonly seen in winter months and was community‐acquired in two‐thirds of cases. Isolation of this organism was associated with a broad variety of upper and lower respiratory 'tract syndromes..Isolates were considered to be of pathogenic significance (all four above criteria satisfied) in 35% of cases and of possible significance (the first and fourth criteria satisfied) iii a further 15% of cases. Isolates were more likely to be of pathogenic significance in older patients and in those with pre‐existing cardiorespiratory disease; however, a number of serious infections were observed in previously‐well children. Expectorated sputum and tracheal aspirates were more likely to yield a clinically significant isolate than nasopharyngeal aspirates. Production of β‐Iactamase was demonstrated in 88% of isolates. Conclusion: B. catarrhalis causes respiratory infection more frequently than is generally appreciated. Isolation of this organism from the respiratory tract had pathogenic significance or possible pathogenic significance in 50% of our patients. If therapy is indicated in patients with respiratory infection caused by this organism, traditional β‐Iactam regimens cannot be relied upon, as shown by the high rate of β‐lactamase production in this study; a tetracycline, erythromycin, a second or third generation cephalosporin, or the combination of a penicillin derivative and β‐Iactamase inhibitor should be considered.