Aerobic gram negative bacterial infections occur commonly in patients with acute leukemia undergoing intensive chemotherapy-induced bone marrow aplasia, and often arise from the gastrointestinal (GI) tract. The value of oral prophylactic norfloxacin (400 mg every 12 hours) in preventing bacterial infections was determined for 68 adults with acute leukemia by a prospective, randomized, double-blind, placebo-controlled trial. The efficacy of norfloxacin administered prophylactically throughout the course of intensive therapy and deep (less than 100 mm/3), prolonged (median 32 day) granulocytopenia against infections arising from the GI tract was demonstrated by its impact on the clinical manifestations of infection, microbiologically documented gram negative bacterial infections, overall antibiotic management, and GI colonization. Although there was no difference with respect to survival, norfloxacin decreased overall morbidity associated with gram-negative infections, was well tolerated, did not impinge on systemic antibacterial or antitumor therapy, and did not predispose to the development of bacteria resistant either to itself or to multiple other antibiotics. The advantages of norfloxacin relative to other oral agents used for GI prophylaxis and the potential beneficial effects of norfloxacin or overall medical cost are discussed. Oral norfloxacin achieves the goals of GI prophylaxis by effectively suppressing early infection arising from the GI tract and inhibiting GI colonization by multiply resistant pathogens that could give rise to late-onset infections during profound chemotherapy-induced bone marrow aplasia.