Biofilm has an important role in the pathophysiology of chronic rhinosinusitis (CRS). The anthrocoanal polyp (ACP) is a distinct entity within the CRS. Objectives The aim of the study was to determine whether there were electron-microscopic signs of biofilm in ACP and in the maxillary sinus in the patients with ACP and in the control group. Furthermore, the existence of the biofilm at the above mentioned localizations was compared with the histopathological findings of the mucosal specimens, to determine status of the mucosa on which the biofilm occurs. Microbial flora in the nasal cavum and in the maxillary sinus was also analyzed. kako bi se utvrdilo stanje sluznice na kojoj se biofilm javlja Study Design Cross-sectional study. Material and Methods A total of 80 individuals of both sexes over the age of 18 were included in the study: 40 patients with ACP and 40 control group members (patients undergoing surgery through healthy sinuses, for another pathological substrate). The bacterial distribution in both their planktonic and biofilm forms and the presence of mucosal inflammation of the maxillary sinus were analyzed. Tissue samples of the ACP part in the maxillary sinus, the ACP part in the nasal cavum and the mucosa of the maxillary sinus were analysed pathohistologically and by scanning electron-microscopic examination. The findings of the pathohistological analysis were grouped according to the Terrier classification of chronic mucosal inflammation of the maxillary sinus. A microbiological swab of the nasal cavum and maxillary sinus was collected from all the patients. Results The research resulted in finding the biofilm in all three localizations in ACP patients and in the mucosa of the maxillary sinus of the control group but statistically significantly less frequently, in 5% of the control group and never in Terrier type 1. The positive microbial swab was found in the statistically significantly greater number of patients with ACP, than in the control group, in both the cavum and the maxillary sinus. The most commonly detected microorganism in both localizations was Staphylococcus aureus, the most common and the most important pathogen in CRS. Conclusion ACP has a significant prevalence of biofilm, confirmed in 95% of cases. Neither the electron-microscopic morphological differences of the biofilm at the three investigated localizations, nor the pathohistological findings of the three mentioned localizations in terms of chronic inflammation differ from each other. Biofilm was also found in control group, i.e. those without clinical and radiological signs of CRS. Furthermore, the study showed the predominance of sterile maxillary sinus mucosa in the control group.