The bacteriological study of sinuses has been investigated in patients with chronic maxillary sinusitis. In obtaining a representative specimen of maxillary sinusitis, various techniques have been employed. The author examined 48 patients (70 sinuses) who had Caldwell-Luc's operation, during the period of May 1981 to August 1982. The material for bacteriological examination has been obtained by swabbing of the secretion and taking the antral mucosa simultaneously during Caldwell-Luc's operation. The following results were obtained. 1) Antibiotics were used preoperatively in 47 sinuses (67%) and not in 23 sinuses (33%). 2) Comparing culture of mucosa with secretion in sinus, same bacteria were identified in 3 sinuses (4%) and different bacteria in 28 sinuses (40%). 3) Staphylococci were the most commonly isolated bacteria (43%). Bacterial isolation in non used group of antibiotics was nearly two times higher rate than antibiotics used group preoperatively. 4) In 44 bilateral sinuses, same bacteria were identified in 2 sinuses (5%) and did not in 18 sinuses (40%), but no growth in 24 sinuses (55%). 5) Isolation of bacteria was not related to allergy, extent of sinusitis and macroscopic appearance of secretion in sinus. 6) In sensitivity test, Staphylococcis showed 100% sensitivity to cephalotin and hemolytic Streptococcis showed 100% sensitivity to erythromycin, lincomycin, chloramphenicol and cephalotin.