The posterior edentulous maxilla is a critical anatomic region for dental implant therapy. Because of severe alveolar bone resorption and maxillary sinus pneumatization, low bone volume is often presented clinically. Although maxillary sinus augmentation has been developed to promote bone reconstruction and oral rehabilitation, complications have been reported. Possible complications include paranasal sinusitis, loss of the graft, and displacement of an implant into the antrum. In this study, we present an observed rare complication of maxillary sinus augmentation, a postoperative maxillary cyst that occurred 10 years after treatment.