The aim of this cross-sectional study was to determine the signs of biofilm in the maxillary sinus of patients with antrochoanal polyps (ACP), and status of the mucosa on which the biofilm occurred. Mucosal samples from maxillary sinus in 40 ACP patients who underwent endoscopic sinus surgery were analyzed histopathologically and by scanning electron microscopy. Results were compared with maxillary mucosa samples of 40 patients without endoscopic and radiological signs of sinus disease. The existence of biofilm and its relation to the degree of histopathological changes according to Terrier classification of chronic mucosal inflammation of maxillary sinus were statistically analyzed. Biofilm was detected in 23 of 40 (57.5%) ACP patients; the incidence was significantly lower in the control group (2/40,5%). Biofilm was not found in type 1 mucosa according to Terrier classification. In conclusion, biofilm showed a significant incidence in the maxillary sinus mucosa of ACP patients (57.5%). Occasionally, biofilm can be found in patients with no signs of sinus disease, but not on histologically normal mucosa. Results of this study support the theory that biofilm formation does not represent the initial stage of the inflammatory process. Key words: Antrochoanal polyp; Biofilm; Maxillary sinus Cilj ovog presjecnog istrazivanja bio je utvrditi postoje li znakovi biofilma u maksilarnom sinusu u bolesnika s antrokoanalnim polipima (ACP) i utvrditi status sluznice na kojoj se biofilm javlja. Uzorci sluznice maksilarnog sinusa 40 bolesnika s ACP-om koji su podvrgnuti endoskopskoj operaciji sinusa analizirani su patohistoloski i skenirajucom elektronskom mikroskopijom. Rezultati su usporedeni s 40 bolesnika bez endoskopskih i radioloskih znakova sinusne bolesti. Analizirana je i statisticki obradena prisutnost biofilma i njegova povezanost sa stupnjem patohistoloskih promjena prema Terrierovoj klasifikaciji kronicne upale sluznice maksilarnog sinusa. Biofilm je pronaden u 23 od 40 bolesnika (57,5%) s ACP-om, a statisticki znacajno rjede u bolesnika kontrolne skupine (2 od 40,5%). Biofilm nije pronaden na sluznici tipa 1 prema Terrierovoj klasifikaciji. Ovo je istrazivanje pokazalo znacajnu incidenciju biofilma na sluznici maksilarnog sinusa u bolesnika s ACP-om (57,5%). Sporadicno se biofilm moze naci i u asimptomatskih bolesnika (bez klinickih, endoskopskih i radioloskih znakova sinusne bolesti), ali ne i na normoplasticnoj sluznici. Rezultati ovog istrazivanja podupiru pretpostavku da stvaranje biofilma ne predstavlja pocetnu fazu upalnog procesa. Kljucne rijeci: Antrokoanaln i polip; Biofilm; Maksilarni sinus