Acute episodes of bronchitis have been shown to be unequally distributed within a population of subjects with chronic bronchitis. Two groups were identified based on incidence of acute bronchitis--subjects who were 'infection-prone' (2-5 infections per year) and those who were 'non-infection-prone' (0-1 infections per year). Minor differences in clinical parameters existed, except for smoking experience. The non-infection-prone group included more current smokers, and the total smoking experience (in 'pack years') was significantly greater in this group. Between-year analysis demonstrated a stability of classification, established after a minimum of two years' prospective observation. Parameters of the host-parasite relationship were assessed in both groups. A significantly greater polybacterial colonization of the oropharynx was observed for chronic bronchitics, both infection-prone (P < 0.0001) and non-infection-prone (P < 0.001), compared with control subjects. Infection-prone chronic bronchitics had significantly greater total bacteria cultured from the oropharynx compared to the non-infection-prone group (P < 0.05); adherence of indigenous microflora to buccal epithelial cells, in particular Gram-positive cocci (P < 0.01) and in vitro adherence of non-serotypable Haemophilus influenzae to buccal cells (P < 0.05) compared with the control and non-infection-prone groups. These studies suggest that an important variation in subjects with chronic bronchitis is the binding capacity of epithelial cells for bacteria, which when increased enhances susceptibility to colonization and clinical infection.