Infection is the most common cause of mortality in neutropenic patients. Although fever does not necessarily mean infection, it must be regarded as its first sign and treated, within hours of its onset, on the basis of probability before a pathogen is isolated. The first-line treatment must cover a wide antibacterial spectrum corresponding to the usual bacteriological flora and to the patient's underlying pathology. The risk of Gram-negative septicaemia in infants and elderly people and the frequency of staphylococcal infections in patients with an indwelling central catheter are well-known. The "best guess" treatment should consist of a third generation cephalosporin, notably ceftazidime, and an antistaphylococcal antibiotic. This treatment should be pursued throughout the period of neutropenia. Due to advances in antibacterial therapy, more aggressive chemotherapeutic regimens can now be prescribed to improve the prognosis of acute blood diseases and of numerous carcinomas.