The first principle of infection prevention in neutropenic patients is to ensure that every effort is made to prevent impairment of the host's defences or any disturbance of the ecological balance of the patient's microbial flora. The second principle is that potential or established sources of infection should be sought and ideally treated before any immunosuppressive therapy is instituted. The third principle is to define the extent to which a particular patient can be expected to benefit from special measures such as protective isolation, sterile or low-pathogen food, decontamination, granulocyte transfusions, passive or active immunization, or antimicrobial prophylaxis aimed at a specific micro-organism such as Pneumocystis carinii or Mycobacterium tuberculosis. A programme for the prevention of infections in neutropenic patients will fail if any of these three principles is ignored.