A study was made of the bacteriology of acute maxillary sinusitis, diagnosis being made on the basis of clinical and radiological examinations. Material for study was taken only from maxillary sinuses with secretion, and consisted of 243 secretion specimens from 171 untreated patients. Bacteriological findings were in good agremeent with other recent studies. Most specimens yielded Diplococcuspneumoniae or Haemophilusinfluenzae or no growth. The findings were analyzed with regard to age and seasonal distribution. Staphyiococcusaureus, which is sometimes taken to indicate the degree of nasal contamination, was present in 10% of cultures. Decreased sensitivity to penicillin was found in 24% of all bacteria isolated, and to tetracycline in 3.5%. Stress is placed on the need for an accurate definition of this condition. The suitability of penicillin for the treatment of acute maxillary sinusitis is questioned.