A quantitative deficiency in polymorpho-nuclear leukocytes, due to aplastic anemia, exposes to infection. The risk is all the greater when the neutropenia is more marked and more lasting. The infections have a different distribution from that commonly observed in normal subjects. There is little inflammatory reaction, no pus formation and bacterial multiplication invades the parenchyma and may create necrosis due to arteriolar obstruction. The prognosis is very bad. For example, pulmonary infections in acute leukemia of adults, have a mortality greater than 75%. Antibiotic treatment and leukocyte transfusions give disappointing results. The prevention of infections has permitted spectacular progress. Nevertheless, the long-term prognosis is not linked to the infection itself, but to the sub-jacent disease responsible for the agranulocytosis. The infections become cured in transient toxic aplasia and in leukemia where chemotherapy permits one to obtain a remission. The infections remain fatal whatever the treatment used if the medullary aplasia is not curable.