There have been few reports on Branhamella catarrhalis as a nosocomial pathogen, and no risk factors for nosocomial infection have been identified. We report 11 cases (mean age 22 months) of nosocomial Branhamella catarrhalis respiratory tract infection in a paediatric intensive care unit (PICU) over a two-year period. There were 2 cases of pneumonia and 9 cases of bronchitis. Branhamella catarrhalis was the sole isolate recovered in 6 cases and was associated with other respiratory pathogens in 5 cases. A case-control study with two age-matched controls per patient (mean age 24.1 months) was undertaken to identify potential risk factors for infection; risk factors identified were the presence of an endotracheal tube (p less than 0.02) and frequent endotracheal tube suction (p less than 0.05). Five of 6 tested strains from PICU patients produced beta-lactamase. DNA preparations of 4 B. catarrhalis isolates from PICU patients revealed no plasmids. B. catarrhalis should be considered a potential nosocomial pathogen.