Bacteremia caused by Aeromonas species occurred in 24 hospitalized patients in a cancer institute during a 13-year period. All but one of these patients had a malignancy (88% had leukemia), and most were receiving chemotherapy for cancer. There was a striking numerical predominance of male patients (82%). Unlike some previously described patients with infections due to this organism, none of these 24 patients had recently been exposed to fresh or salt water or to fish. The source of the infecting organism was thought to be endogenous--i.e., from patients' own gastrointestinal tracts. The clinical presentation of sepsis caused by this organism was nonspecific, except that ecthyma gangrenosum occurred in several patients. The overall mortality rate was 28%. The combination of an aminoglycoside and a cephalosporin is appropriate therapy for bacteremia caused by Aeromonas species.