A 44-year-old Japanese man experiencing repeated nosebleeds was diagnosed as having a blood boil in the maxillary sinus. Preoperative MRI showed a tumor in the maxillary sinus. Because this was suspected of being malignant, a biopsy was planned. Surgery was done endoscopically, however, with the tumor excised with minimal blood loss. Histopathological studies revealed blood including a mixture of inflammatory cells and fibrin deposits, and the tumor was thus diagnosed as a blood boil. We discuss the case with reference to indications for endoscopic sinus surgery for maxillary sinus blood boils and statistics from the literature.