Organized hematoma of the maxillary sinus is rare. Its pathogenesis is still not clear, and it appears to have multiple causes. We report the clinical, clinicopathologic, and immunohistochemical findings in 5 patients-3 men and 2 women, aged 19 to 68 years (mean: 40.2)-with pathologically proven organized hematoma of the maxillary sinus. In all 5 cases, we were able to successfully perform curative treatment via endoscopic sinus surgery. Our findings suggest that most organized hematomas are angiomatous. Immunohistochemical analysis identified the presence of proliferative activity.