To test the value of pefloxacin for the prevention of infections in patients with chemotherapy-induced neutropenia, oral pefloxacin plus vancomycin (PV) (n = 76) or gentamicin, colistin sulphate and vancomycin (GCV) (n = 74) were administered in a randomised double-blind study. Infections were significantly less severe in the PV than in the GCV group. Patients receiving PV had significantly fewer episodes of bacteraemia and central venous line infections than patients treated with GCV. Gram-positive and gram-negative infections were significantly less frequent in patients receiving PV, because of fewer infections with Staphylococcus species and enterobacteriaceae. Stool culture detected significantly more gram-positive organisms in the PV group and more gram-negative organisms in the GCV group. Thus, PV was more efficacious than GCV for the prevention of gram-positive and gram-negative infections in the neutropenic patients, despite lower efficacy in eradicating gram-positive organisms from the lower intestinal tract.