Repeated aspirations of sinus secretions by endoscopy was performed in five patients over a period of 34-50 days and, ultimately, surgical drainage was done in three who presented with acute maxillary sinusitis that did not respond to antimicrobial therapy and became chronic. The aspirates were cultured for aerobic and anaerobic bacteria. Most of the bacteria isolated from the first culture were aerobic or facultative bacteria: Streptococcus pn eum oniae (three isolates), Haemop hilus injluenzae non-t ype-b (two) and Moraxella catarrhalis (one). Three of these cultures yielded bacteria that were resistant to the antimicrobial agents prescribed for treatment. Failure to respond to therapy was associated with the emergence of resistant aerobic and anaerobic bacteria in subsequent aspirates. These organisms included Fusobacterium nucleatum, pigmented Prevotella and Porphyromonas spp. and Peptostreptococcus spp. Eradication of the infection was achieved in all instances following the administration of antimicrobial agents effective against these bacteria, and in three instances by surgical drainage. This study illustrates the microbial dynamics of maxillary sinusitis that did not respond to antimicrobial therapy.