The blood boil is a benign hemorrhagic pseudotumor that usually arises in the sinonasal tract, particularly the maxillary sinus and blood boils in the frontal sinus are very rare. We experienced a rare case of a blood boil in the frontal sinus. A 55-year-old female visited our hospital because of a swelling in her forehead and lateral deviation of the left eye. CT and MRI showed a large mass extending from the left frontal sinus to the orbital roof with bone erosion. Because of poor improvement with conservative medical treatment, we performed a frontal sinus operation via the transcranial approach and removed the tumor. Histopathologically, the tumor was composed of necrotic tissue with hemosiderin deposition and no malignant tissue, and it was diagnosed as a blood boil in the frontal sinus. No recurrence has been seen for about 1 year after the operation.
