Conflicting data exist about the presence of bacteria in healthy maxillary sinus cavities. This study was designed to determine the bacterial flora and to quantify the level of bacterial presence in healthy maxillary sinus cavities. Subjects included 34 patients undergoing Lefort I osteotomy for orthognathic surgery. All patients were preoperatively evaluated by a questionnaire and a complete physical examination including sinus endoscopy. Our exclusion criteria were presence of sinonasal symptoms, asthma, antibiotic treatment in the past 3 months, treatment with local steroids, previous sinonasal surgery, traumatic surgery, and an abnormal CT scan or sinus endoscopy. Washes were obtained from maxillary sinuses before surgery through an antral puncture. The sinus was irrigated with sterile saline followed by aspiration with a syringe attached to the trocar. Basic sterility rules were rigorously applied. Specimens were transported to the laboratory in an air-free syringe. Time between collection of materials and inoculation of the specimen did not exceed 15 minutes. Specimens were inoculated for aerobic and anaerobic organisms. After applying the selection criteria, 14 patients (28 sinuses) remained. Eight (57.1%) were men with a mean age of 22.7 years; 82.14% of the specimens were sterile. Bacterial organisms were recovered in only four patients with two different coagulase-negative staphylococci in the same patient: one in each sinus with 200 UFC/mL in the left sinus and 10 UFC/mL in the right sinus, one Citrobacter fundii (70 UFC/mL) and two polymorphic floras. This descriptive study shows the large predominance of sterile maxillary sinus cavities in asymptomatic adults with endoscopically normal mucosa.