This is a report of a case of primary mucocele occurring in the right maxillary sinus, which is supposed to be rare. A 30-year-old female patient visited to the Ewha Womans University Hospital on Feb. 26, 1965 with complaint of a palpable mass in the right cheek. The external examination revealed that a walnut sized, moderately hard, and round shaped mass, which has slight fluctuation, was palpated on the right maxillary region. However no tenderness could be demonstrated. Aspiration of the contents disclosed straw colored mucoid fluid, and megative finding were detected in the nasal cavity by anterior rhinoscopy. X-ray study of the maxillary sinus showed slight cloudiness over the right maxillary sinus without definite evidence of bone destruction. Under suspiction of a premaxillary cyst, an exploratory operation (Caldwell-Luc's operation approach) was performed and found that a large mucocele of the maxillary sinus which involving the anterior and medical bony wall of the sinus. The whole mucosal sac was carefully separated and removed, and a wide enough counter opening was made. Post operative course was satisfactory and no undesirable sequela occurred up to date.