The oropharynx and intestinal microflora of patients from intensive care and infectious disease units of an university hospital were compared to that of healthy individuals in order to evaluate the influence of antibiotic therapy on the colonization with enterobacteria and antimicrobial resistance patterns. Enterobacteria were recovered from 70% of the throat specimens; 90% of these individuals were under antimicrobial therapy. The antibiotic resistance patterns were suggestive of plasmid-mediated resistance. More than 20% of the strains were resistant to fosfomycin, ampicillin, chloramphenicol, tetracycline or trimethoprim-sulfamethoxazole. Approximately 50% of resistant bacteria showed resistance to at least four antibiotics.