Antimicrobial resistance among bacteria has arisen ever since antimicrobial agents were introduced in the clinic. Unfortunately, it seems that resistance is now emerging at a more rapid rate than ever before, as a consequence of the widespread use of antimicrobial agents. The spread of these multiresistant microorganisms is an increasing threat in many countries. The human intestinal flora is a huge potential reservoir of resistant microorganisms. Antimicrobial resistance in clinical isolates may cause serious infections and treatment failure, and lead to the use of higher doses or more toxic alternative drugs. This study was planned so as to compare the effects of hospitalization and antibiotic usage on the aerobic intestinal flora and included 43 hospitalized adult patients without any previous history of hospitalization and antibiotic usage during the last 30 d. Patients were divided according to their antimicrobial therapy, into treated and untreated groups. The individual use of antimicrobials was recorded. Antibiotic usage was found to be more effective on the aerobic intestinal flora compared with hospitalization without such medication.