During the 25-year period from 1972-1996, 360 episodes of bacteremia occurred in 248 (45%) of a total of 548 patients with acute leukemia on our hematology ward, with the rate of occurrence remaining almost unchanged. Patients with acute nonlymphocytic leukemia, however, had a significant decrease in the incidence of bacteremia during the last 5-year period. Gram-negative bacilli decreased in relative frequency from 64% for the first 15-year period to 40% for the last 5-year period, whereas gram-positive cocci increased from 24% to 51%. Escherichia coli and Enterobacter cloacae somewhat decreased in frequency, whereas coagulase-negative staphylococci and streptococci had considerable increases. In contrast, Pseudomonas aeruginosa was isolated at a relatively constant frequency, forming one of the top two pathogens throughout the study period. Despite the shift toward gram-positive cocci, therefore, P. aeruginosa remains a key organism in considering the initial empiric antibiotic regimen and infection prevention during neutropenia.