The aim of this study was to prospectively examine the effect of prophylactic antibiotic use on the development of respiratory infections and on the worsening of symptoms after diagnostic fiberoptic bronchoscopy procedures. This study was an open-label, multicenter, controlled, clinical trial. Patients were alternately assigned to a group given prophylactic antibiotics after bronchoscopy (prophylaxis(+) group) and a group not given antibiotic prophylaxis after bronchoscopy (prophylaxis(-) group), and they were followed-up for 1 week. 158 patients were assigned to the prophylaxis(-) group and 153 to the prophylaxis(+) group. Therapeutic antibiotic administration was needed in 3 patients (1.90%) in the prophylaxis(-) group and 5 patients (3.27%) in the prophylaxis(+) group (risk ratio 1.014, 95% confidence interval 0.978-1.052; p=0.446). Worsening of symptoms after bronchoscopy occurred in 57.6% of all patients by day 7, but no significant differences were observed between the 2 study groups. Prophylactic antibiotic use after bronchoscopy did not prevent the development of infectious events and worsening of symptoms, suggesting that prophylactic antibiotics might not be necessary for routine diagnostic bronchoscopic procedures.