ORGANISMS of low pathogenicity often cause serious infections in patients with acute leukemia. Septicemia is a major problem and often proves fatal. Although usually only one organism is recovered from the blood, the isolation of multiple organisms is not infrequent. We have reviewed our experience with 54 episodes of multiple organism septicemia in 52 patients with acute leukemia. Methods Four hundred twenty patients with acute leukemia were admitted to the National Institutes of Health from October 1953 to August 31, 1963. The clinical records and autopsy protocols of these patients were examined. Patients were included in the study if more than one organism was grown from their blood in a single culture or from separate cultures during any two-week period, but not if they contained obvious contaminants such asStaphylococcus albus, diphtheroids, α-hemolyticStreptococcusor group DStreptococcus. Postmortem heart blood cultures were not included unless a morphologically consistent site