Most maxillary sinus papillomas are confirmed when they have extended beyond the nasal cavity and are rarely found while localized in the maxillary sinus. We experienced two cases of localized papilloma in the maxillary sinus. Case 1 was a 69-year-old man with a localized left maxillary sinus lesion detected during a routine imaging examination. As the lesion was likely to be papilloma, we recommended that the patient undergo diagnostic surgery, which he refused. He experienced bloody rhinorrhea 1 year and 9 months after the first visit, and computed tomography (CT) showed increased lesions and bone destruction. Histological examinations revealed squamous cell carcinoma ex inverted papilloma. He died 5 years after the first visit. Case 2 was a 46-year-old woman in whom positron emission tomography/CT showed a localized right maxillary sinus lesion. Tissue biopsy results indicated oncocytic papilloma. Endoscopic resection was performed later. On an imaging examination, sinonasal papilloma was determined accidentally to be a localized lesion of the maxillary sinus. A detailed interpretation of the CT scan was useful in estimating sinonasal papilloma. Tissue biopsy or diagnostic surgery should be performed when sinonasal papilloma is suspected during appropriate image evaluation.