In a 48-year-old woman and a 35-year-old man, a sinusitis maxillaris resulting from overextension of root canal filling material after an endodontic treatment of a maxillary molar could be diagnosed through computed tomography. The woman complained of nasal congestion and in the man 2 radiopaque abnormalities in the maxillary sinus were discovered coincidentally. The corpora aliena were removed surgically. Root canal filling materials containing zinc oxide or hydrocortisone may cause aspergillosis of the maxillary sinus because they encounter growth of Aspergillus species. In case of maxillary sinusitis, surgical removal of overextended root canal filling material from the maxillary sinus is indicated. No treatment consensus consists for cases without maxillary sinusitis symptoms.