Actinomycosis is an infection caused by anaerobic bacteria, primarily from the genus Actinomyces, which normally colonize the several regions including the mouth. Disruption of mucosa may lead to infection of virtually any site, but reports of involvement of the sinuses are rare. We report a case of an actinomycosis infection in the unilateral maxillary sinus. A 47-year-old female visited our hospital with a complaint of mild pain of left buccal region. Computed tomography (CT) revealed that the left maxillary and anterior ethmoidal sinus cavities were opacified along with a calcified fragment located close to the natural ostium. Thus, we provisionally diagnosed as a fungal sinusitis. She underwent trans-nasal endoscopic sinus surgery. The sinuses were opened and the caseous material was removed. The histopathological examination suggested an actinomycosis, but not fungal, infection. The patient’s postoperative course was uneventful. No evidence of recurrence has been seen over the 30 months of the postoperative follow-up period. In case of the aggressive actinomycotic sinusitis, extension into the adjacent organs could be occurred. We should be aware that sinusitis of actinomycosis infection could progress in patients with risk factors such as diabetes and immunodeficiency
