In a prospective study, patients with the diagnosis of chronic ethmoid sinusitis were evaluated microbiologically by using biopsy specimens of the ethmoid sinus mucosa. Microbiology cultures were performed on 94 specimens from 59 patients. Staphylococcus aureus and members of the family Enterobacteriaceae were the most frequent classical pathogenic bacteria isolated. Coagulase-negative staphylococci were the most common overall isolates. Streptococcus pneumoniae and Haemophilus influenzae were infrequent isolates. No anaerobes, viruses, or Chlamydia trachomatis organisms were identified. Results of this study showed organism isolation frequencies different from those found in other studies of chronic sinusitis reported in the literature. The predominance of S. aureus and members of the family Enterobacteriaceae could have an effect on the antimicrobial therapy for chronic ethmoid sinusitis.