Trends in the detection of causative pathogens and changes in bacterial counts in patients with sinusitis treated between January 1989 and December 1993 were investigated. In adult patients with chronic sinusitis, Staphylococcus aureus (S. aureus), coagulase negative staphylococci (CNS), Streptococcus pneumoniae (S. pneumoniae), Corynebacterium sp., Haemophilus influenzae (H. influenzae), and Moraxella catarrhalis were often isolated while Pseudomonas aeruginosa (P. aeruginosa) and anaerobic bacteria were detected in 2.4% and 5.3% of patients, respectively. The bacteria isolated from adult patients with acute sinusitis and pediatric patients with either acute or chronic sinusitis were somewhat different from those of adult chronic sinusitis. No bacteria could be isolated from 5.8% of adult chronic sinusitis patients, 8.1% of adult acute sinusitis patients, and 3.1% of pediatric sinusitis patients. The detection rate for anaerobic bacteria has been rising in chronic sinusitis patients owing to improved detection techniques in recent years, while there has been no appreciable change in the isolation rate for other types of bacteria. When the pathogenicity of isolated bacteria was determined based on the amount of bacterial colonization it was found that P. aeruginosa, S. pneumoniae, H. influenzae, and S. aureus were significant as causative pathogens in sinusitis, while CNS.