Thirty-six patients with severe aplastic anemia or acute leukemia undergoing bone marrow transplantation or intensive hematologic treatments were randomized to receive gentamicin (1 mg/kg/iv q8h) or tobramycin (1 mg/kg/iv q8h) in combination with carbenecillin (500 mg/kg/day i.v. in 4 doses) and cephalotin (200 mg/kg/day i.v. in 6 doses), at the onset of fever during granulocytopenia. Patients in the gentamicin group were treated for a total of 541 patient days (minimum 5, maximum 106), and patients in the tobramycin group for 426 patient days (minimum 9, maximum 48). All patients received oral decontamination, which included the amino glycoside (either gentamicin or tobramycin) given intravenously. This study showed that 1) no patient had an increase in creatinine level above 1.3 mg % and no patient developed renal failure, 2) there was no difference between gentamicin and tobramycin in the efficacy of treating febrile episodes and/or major infections, 3) prolonged intravenous administration (up to 48 consecutive days in the tobramycin group and 106 consecutive days in the gentamicin group) was well tolerated and effective in treating fever of unknown origin in granulocytopenic patients: 45 of 72 febrile episodes resolved while the patients had a granulocyte count below 500/mm3.