The main objectives of this study were to estimate the frequency of chronic maxillary sinusitis of dental origin, and to evaluate the microbiology of odontogenic and non-odontogenic chronic maxillary sinusitis. Aspirates from 59 patients with chronic maxillary sinusitis (47 non-odontogenic, 12 odontogenic), collected during a 3-year period, were microbiologically processed for aerobic and anaerobic bacteria. Moreover, antimicrobial susceptibility was evaluated in the isolated bacteria. In this study, 20 % of chronic maxillary sinusitis cases were associated with a dental origin, and sinus lift procedures were the main aetiological factor. Our microbiological findings showed that all specimens from chronic maxillary sinusitis were polymicrobial. Sixty aerobes and 75 anaerobes were recovered from the 47 cases of non-odontogenic sinusitis (2.9 bacteria per specimen); 15 aerobes and 25 anaerobes were isolated from the 12 patients with odontogenic sinusitis (3.3 bacteria per specimen). The predominant aerobes were Staphylococcus aureus (27) and Streptococcus pneumoniae (16), while the more frequent anaerobes were Peptostreptococcus species (31) and Prevotella species (30). Haemophilus influenzae and Moraxella catarrhalis were absent in sinusitis associated with a dental origin. Overall, 22 % of Staphylococcus aureus isolates were oxacillin-resistant, and 75 % of Streptococcus pneumoniae isolates were penicillin-resistant and/or erythromycin-resistant; 21 % of anaerobic Gram-positive bacteria were penicillin-resistant, and 44 % of anaerobic Gram-negative bacteria were β-lactamase-positive. Vancomycin and quinopristin–dalfopristin had the highest in vitro activity against Staphylococcus aureus and Streptococcus species, respectively; amoxicillin–clavulanate and cefotaxime showed the highest in vitro activity against aerobic Gram-negative bacteria; and moxifloxacin, metronidazole and clindamycin were the most active against anaerobic bacteria.