Aztreonam (Az), a minimally absorbable monobactam antibiotic, was compared to colistin plus neomycin (CN), for intestinal decontamination during Bone Marrow Transplantation (BMT) in a controlled study. Thirty-four consecutive patients were randomized in two groups and evaluated for number of febrile episodes, days of fever, fecal cultures and clinical symptoms related to infections or colonizations. No significant differences were observed suggesting that Az is at least as effective as the CN regimen and may be considered as an alternative approach for intestinal decontamination in BMT patients.