Background In chronic nonallergic sinusitis, it is often assumed that bacteria invade the sinus mucosa where the inflammatory condition begins and is maintained. However, the bacterial presence in a normal or moderately damaged epithelial layer has never been proved in biopsy studies. Methods In this study, mucosal samples from six consecutive patients with chronic sinusitis were examined. Transmission electron microscopy was used and the presence of bacterial invasion and formation of phagosomes containing bacteria as a marker of host response were studied. Results Phagocytosis of bacteria was observed in the sinus mucosa in samples from only one patient. In the other five patients, no signs of phagocytosis were seen. Conclusion Based on these results, we concluded that in chronic sinusitis, bacterial invasion in sinus mucosa is not an obligatory phenomenon.