The quantitative evaluation of Pseudomonas aeruginosa adhesion to the respiratory epithelium in culture by scanning electron microscopy observation required the evaluation of the filling rate of bacterial aggregates. We used classical stereological methods. The bacterial aggregates were found to be occupied by bacteria in 50% of their total volume. This result will help us to understand the aggregation process of Pseudomonas aeruginosa and to analyze the effect of molecules used for preventing bacterial adhesion to the respiratory epithelium.