The aim of this case report was to present clinical, radiologic, and histologic healing outcomes following functional endoscopic sinus surgery (FESS) to manage maxillary sinusitis after lateral sinus augmentation. Three patients who underwent lateral sinus augmentation developed maxillary sinusitis after 3 to 4 weeks. Pharmacologic interventions were performed, but the patients' symptoms did not resolve. The patients were referred to an otorhinolaryngologist and underwent FESS without the removal of bone substitute material or implants. The patients' symptoms disappeared completely after FESS. The implants were osseointegrated successfully and have functioned well to date (between 4 months and 7 years after definitive prosthesis delivery). No pathologic change was noted clinically or radiologically after FESS. However, biopsy specimens demonstrated some new bone formation with varying degrees of fibrotic change and tissue density, which is not compatible with the clinical and radiologic observations. FESS without intraoral debridement may be sufficient to resolve maxillary sinusitis after lateral sinus augmentation; this may not lead to favorable histologic healing in the augmented sinus. Keywords: case report, dental implant, endoscopic sinus surgery, sinus augmentation, sinusitis