A ''quasi-experimental'' trial was carried out to investigate the effect of three antimicrobial regimens on oral and fecal yeast colonization in patients with hematologic malignancies. Fifty-four patients received ciprofloxacin and oral amphotericin B (group 1); 45 received ceftazidime, amikacin, vancomycin, and oral amphotericin B (group 2); and 30 received ceftazidime, amikacin, vancomycin, and intravenous amphotericin B (group 3). The oral yeast isolation rate showed a decrease in group 1 (from 59.3% to 40.7%) and group 3 (from 56.7% to 46.7%), and a marked increase in group 2 (from 51.1% to 84.4%). All the groups showed a reduction in their fecal yeast isolation rate. An overgrowth of Candida parapsilosis, C. krusei, and C. tropicalis was observed in all the groups, but it was much higher in group 2. Our findings provide evidence that ceftazidime, amikacin, and vancomycin, given with oral amphotericin B, induce an overgrowth/persistence of Candida species in the mouth and gut, which might be attributable to inclusion of vancomycin. Treatment with intravenous amphotericin B has at least the capacity of counterbalancing yeast proliferation induced by that antibacterial regimen.