Background: Patients with chronic rhinosinusitis (CRS) often remain symptomatic after technically proficient functional endoscopic sinus surgery. Current hypothesis indicates biofilms may contribute to the persistence of infection. However,few studies showed biofilms in postoperative patients. This study was designed to identify bacterial biofilms on postoperative mucosa, as well as to investigate the healing of sinus mucosa after surgery. Methods: After intraoperative mucosa was obtained for assessment of biofilms, 27 patients were followed up for 6 months. Postoperative medications and symptoms were recorded. As indicated by endoscopic evaluation, biopsy specimens of postoperative edema, scar, or adhesion were obtained. Samples were prepared for scanning electron microscopy (SEM) and hematoxylin and eosin (H&E) staining. Results: Fifteen postoperative samples were taken from the 20 patients with intraoperative biofilms. Under SEM, postoperative biofilms were identified in 416 scar samples and 519 edema samples. There was no significant difference in biofilm presence between samples of scar and edema. Microcolonies were also identified on postoperative scar under H&E staining. The presence of intraoperative and postoperative biofilms was correlated with the severity of preoperative Lund-MacKay computed tomography score and postoperative Lund-Kennedy endoscopic score. Compared with intraoperative samples, postoperative samples from the same nine patients significantly recovered from ciliary damage, metaplasia, and basement membrane thickness. Postoperative cultures were positive in samples with and without postoperative biofilms. Conclusion: Biofilms persist after treatment, and may cause the unfavorable outcomes of surgery for CRS. The mucosa with biofilms can recover after surgery. Apparent bacterial plaque can be identified by H&E staining. (Am J Rhinol Allergy 23, 506-511, 2009; doi: 10.2500/ajra.2009.23.3376)