Forty consecutive neutropenic patients with acute leukemia receiving oral ciprofloxacin (500 mg twice daily) and ketoconazole (200 mg daily) for selective intestinal decontamination were compared retrospectively with 33 comparable patients treated with polymyxin E (1,500,000 U x 3/day) and nystatin (1,000,000 U x 3/day). The incidence of febrile episodes was slightly lower in ciprofloxacin treated patients (87.5% vs 100%). No gram-negative sepsis was observed in this group compared with seven cases in patients receiving polymyxin E (p less than 0.01). Furthermore, eight patients in ciprofloxacin group (20%) had gram-positive sepsis, compared with five (15.5%) in the polymyxin E group. The incidence of documented fungal infections was similar in the two groups. Ciprofloxacin appears to be an effective agent for the prevention of gram-negative infections in granulocytopenic patients with acute leukemia, but may contribute to a shift in the type of infections in these patients towards those caused by gram-positive microorganisms, intrinsically fairly sensitive or with acquired drug resistance.