The author had studied to find any clinical characteristics between the unilateral (140 cases) and bilateral (470 cases) maxillary sinusitis. Those patients had been treated at our department during the 3 year period from Jan. 1976 to Dec. 1978. The results were as follows; 1) The frequency of occurrence of bilateral maxillary sinusitis had higher tendency than that of unilateral maxillary sinusitis in age group of first decade, but that of unilateral maxillary sinusitis had higher tendency than that of bilateral maxillary sinusitis in age group of second decade and over fifth decade. 2) The duration of unilateral maxillary sinusitis was usually shorter than that of bilateral maxillary sinusitis. 3) The occurrence of deviation of the affected side in septal deviation had higher frequency in unilateral maxillary sinusitis than in bilateral maxillary sinusitis. 4) The occurrence of hypertrophy of conchae had similar frequency in unilateral and bilateral maxillary sinusitis, and that of nasal polyp had higher frequency in bilateral maxillary sinusitis as twice as in unilateral maxillary sinusitis. 5) In the cases of unilateral maxillary sinusitis, the size of maxillary sinus was usually different from each other, and unilateral maxillary sinusitis had been occurred more in smaller sized one.