Aim: To perform a comparative bacteriological examination of a material, taken from the alveolus of an extracted tooth with chronic inflammatory disease and a material from the maxillary sinus at the closure of oroantral communication, performed immediately after extraction. Background: Odontogenic maxillary sinusitis is a common problem and a well-known condition in the dental practice. This type of maxillary sinusitis differs in its pathophysiology, pathomorphology, microbiology, diagnosis and treatment from the other types of maxillary sinusitis. Chronic inflammatory processes affecting the periodontium and periodontium of the teeth can often reach the maxillary sinuses and thus cause odontogenic maxillary sinusitis. Methods: To perform the task assigned, microbiological samples were prepared, taken intraoperatively from the inflammatory focus around the causative tooth and from the altered mucosa of the maxillary sinus. After taking into account the results of the microbiological examination, a comparative analysis of the microflora isolated from the two sites was performed. This was done in order to prove its identity and to look for the relationship between the two pathological processes. To accomplish the task, we selected 27 patients who had molar teeth extracted, not subjected to conservative treatment, whose roots protrude into the maxillary sinus or stand no more than 3 mm away. In patients where a direct contact with the maxillary sinus was made, we used two sterile swabs to take the microbiological sample – first from the alveolar walls of the extracted tooth and then through the communication made. In other patients, we created such a communication with a fissure burr ourselves, after we had already taken a sample from the alveoli of the extracted tooth. The hole size was no more than 5 mm. Results: When comparing the samples of each patient, taken from the alveoli of the extracted tooth and the pathologically altered mucosa of the maxillary sinus, we obtained the following results – compared to the frequency distribution, we can see in the following table – a match of 88.9% of all cases and in a little over 11% of the cases where not match is found between the samples. Conclusions: When the height of the available bone in the distal parts of the upper jaw between the apexes of the teeth and the floor of the maxillary sinus is below 3 mm, the risk of ascending of the inflammatory process from the periodontal tissues to the maxillary sinus is quite high – 88.89%.