A case of a large compound odontoma in the right maxillary sinus causing acute maxillary sinusitis is reported. Seven large antral odontomas have been previously reported, none with the typical findings of acute sinusitis. Radiographs should always be considered in cases of recurrent maxillary sinusitis or pain, and the general practitioner should familiarize himself with lesions of the maxillary sinus that might be revealed in radiographic examination. Surgical excision is the preferred treatment for all odontomas, with no expectancy of recurrence. The odontoma described here was associated with an unusual bonelike capsule or cyst covering its superior surface, which necessitated careful exploration and dissection.