The role of adherence in the pathogenicity of Branhamella catarrhalis in lower respiratory tract infection in the elderly was investigated. Differential adherence of this organism to buccal epithelial cells of elderly inpatients, outpatients, and young controls was measured. The mean number of bacteria adherent per cell was 36.9 ± 11.2, 40.2 ± 6.1 and 16.5 ± 12.8 for inpatient and outpatient elderly and young controls respectively (significantly different p < 0.01, Student’s t test). This significantly higher level of adherence in the elderly subjects suggests that increased adherence may contribute to the incidence of respiratory tract infection due to B. catarrhalis seen in this g