The role of infection in the deaths of 199 children with leukemia from 1962 through 1969 was studied. Infection was the primary cause of death in 89 patients and associated with other complications in 66. Forty-four of the children were free of infection at the time of death. Pseudomonas aeruginosa, Escherichia coli, Staphylococcus aureus, and Candida albicans were the most frequently encountered etiological agents. Topographically, sepsis, pneumonia, and enterocolitis accounted for most of the infections. Fever was usually of infectious etiology and was not suppressed by chemotherapy. An absolute neutrophil count of less than 500/cu mm was the herald of serious infection. The incidence of mycotic infections increased in the later years and could be related to prolonged survival of leukemic patients. Pneumocystis carinii pneumonitis occurred most frequently with patients in remission.