Sixty-five cancer patients pretreated with chemo or radiotherapy, with granulocytopenia less than 1000/mm3 and without fever, were entered into this study: 30 of them were submitted to prophylaxis with norfloxacin while the remaining 35 patients were used as a control group. 20% of the treated subjects versus 68.6% of the controls presented a subsequent infection (P less than 0.001), the lung representing the most frequent site of the infectious disease in both groups (3/6 and 14/24 respectively). These data strongly suggest the use of norfloxacin as an effective prophylactic drug in nonfebrile, granulocytopenic cancer patients, especially as far as gram-negative infections are concerned. Because of the high prevalence of lung cancer in the patients of our study, and a related prevalence of lung infections, at the present time, a wider use of this antibiotic in every kind of solid tumor cannot be generalized.