Branhamella catarrhalis has been implicated previously as a cause of bronchopulmonary infections. Sputum Gram's stain and culture results suggesting significant infection with beta-lactamase-producing strains of B. catarrhalis were correlated with a retrospective chart review of eight pediatric and ten adult patients. Preexisting pulmonary disease was observed in 12 patients; 5 had a history of aspiration; and 13 were intubated. Clinically, ten patients had pneumonia, five had bronchitis, and three manifested no disease. Only three sputum specimens grew a pure culture of B. catarrhalis, and six specimens yielded B. catarrhalis in the presence of normal upper respiratory flora. Analysis of broth microdilution susceptibility test results showed that 90% of the strains were inhibited at the following minimum inhibitory concentrations (MICs90): ampicillin, 8 micrograms/mL; cefotaxime, 0.5 microgram/mL; cefoxitin, 0.5 microgram/mL; cephalexin, 4 micrograms/mL; cephalothin, 8 micrograms/mL; chloramphenicol, 1 microgram/mL; clindamycin, 4 micrograms/mL; erythromycin, 0.25 microgram/mL; methicillin, 16 micrograms/mL; mezlocillin, 16 micrograms/mL; moxalactam less than or equal to 0.6 microgram/mL; penicillin, 16 micrograms/mL; piperacillin, 8 micrograms/mL; tetracycline, less than or equal to 0.3 microgram/mL; and trimethoprim/sulfamethoxazole, 1.6/30 micrograms/mL. Therapy may have been adequate in only eight (44%) of the cases. However, all but four of the patients, who died of unrelated causes, exhibited resolution of disease. The data indicate that Gram's stain and culture results of sputum specimens suggesting B. catarrhalis bronchopulmonary infection should be interpreted with caution by clinicians.