The purpose of the study was to describe the architecture of accretions occurring in endotracheal tubes used in adults by using a conservative procedure and transmission electron microscopy. The study included 12 tubes for which microbiological data of the tracheobronchial secretions were available. Observations were performed on inducted areas of the lumen. All tubes were covered with a several micrometers-to several millimeters-thick layer of mucus. The layers displayed stratified structures and showed granulations, neutrophils or cellular elements. When bacteria were seen, they made no contact with the polymer. This data suggest that adherence properties of bacteria towards the polymer were not involved at these stages of colonization and that a bacterial biofilms is a rare opportunity.