Because of the persistently high mortality from sepsis in cancer patients, a retrospective study was designed to identify the causative organisms and to determine the factors affecting the outcome of sepsis. A total of 84 episodes of septicemia in 61 children with cancer were studied. The more frequently isolated organisms were: Staphylococcus aureus (21.4%); Escherichia coli (18%); Klebsiella (7.1%); Pseudomonas (6%); and Bacteroides fragilis (6%). Other isolates included Proteus, Serratia, Acinetobacter, hemolytic Streptococcus, and Bacillus cereus. In 10.7% of septic episodes, mixed bacterial infections were documented. Twenty-four (28.6%) resulted in death; in 13 (54%) death occurred within 24 hours after admission. The fatality rate was high in cases associated with absolute polymorphonuclear leukocyte counts of less than 100/cu mm, in neoplastic relapse, and when infection with gram-negative, anaerobic and mixed bacterial flora occurred.