Several previous authors have examined the association between infection and acute leukemia, especially at the time of death, but none has assessed this problem at the time of diagnosis and initial hospitalization. We have undertaken a retrospective review of cases of acute leukemia diagnosed at the affiliated hospitals of the State University of New York at Buffalo. Results suggest that bacteriologic findings in this population initially resemble those in the general outpatient population. Gram-negative bacilli, especially Pseudomonas aeruginosa, appear as important pathogens after the first week of hospitalization. A statistically significant association with prolonged granulocytopenia and use of antibiotics develops during this course. As a consequence of these data, we recommend careful culture and clinical delineation of the early infection, choosing the narrowest spectrum of antibiotic coverage appropriate to the infection present as data evolve.