Background: Bacterial adherence to buccal epithelial cells in chronic smokers has been reported to increase in vitro. However, it is unclear whether such adherence increases in hosts without epithelial changes linked to chronic smoking. Objective: This was an in vivo investigation of whether or not there was a change in the Streptococcus pneumoniae adherence to buccal epithelial cells of rats exposed to different periods and degrees of environmental cigarette smoke. Method: Eighteen rats were divided into acute and chronic exposure groups, each containing 9 rats. Both groups were further divided into 3 subgroups each, which were exposed to low or high levels of cigarette smoke, or none at all. After the administration of smoke, S. pneumoniae (ATCC 49619) was inoculated into the buccal mucosa. One hour later, buccal epithelial cell samples were taken and the number of bacteria adhering to 40 buccal epithelial cells was counted. Pre- and postinoculation values and the groups administered smoke and the control groups were then compared. Results: A significant increase in Gram-positive diplococci adherence was determined in all groups after S. pneumoniae inoculation. S. pneumoniae was observed to adhere to buccal epithelial cells in all the control group rats and in those in the groups exposed to cigarette smoke. Pneumococcal adherence in the chronic exposure subgroups increased approximately 4-fold with low smoke exposure (p < 0.004) and approximately 7-fold with high smoke exposure (p < 0.0005). Conclusion: S. pneumoniae adherence was significantly high in rats chronically exposed to a high level of cigarette smoke, but not in the acute exposure groups. Copyright (c) 2007 S. Karger AG, Basel.