Two hundred eighty-seven episodes of septicemia which occurred in patients with hematological disorders between 1980 and 1993 were examined according to respective underlying diseases. The diagnosis of acute myelogenous leukemia (AML) was made in 155 patients, acute lymphocytic leukemia (ALL) in 45, chronic myelogenous leukemia (CML) in 29, malignant lymphoma in 36, adult T-cell leukemia (ATL) in 7, multiple myeloma (MM) in 8 and aplastic anemia (AA) in 7. Three hundred and two strains were isolated from 287 patients. Fifty two point three percent of the total isolates were gram-negative bacilli, 26.8% were gram-positive cocci, 17.2% were fungi and 3.6% were anaerobic bacteria. In ALL patients gram-positive cocci accounted for 42.0%. This rate was significantly higher than in other disorder. Additionally, oral mucositis or gingivitis was evaluated as clinical background in 36.1% of ALL cases. Forty-seven point two percent of organisms which caused septicemia in ALL patients were isolated from surveillance cultures of the throat just before the onset of septicemia. These data suggested that in ALL cases microbiological organisms more frequently invaded through injuries of oral mucosa. In ATL, CML, MM and AA patients, fungi accounted for more than 25% of causative organisms. The most common organism of all of the strains was Pseudomonas aeruginosa (21.9%), but in ATL and MM patients Escherichia coli was more common than P. aeruginosa. At the onset of the septicemia, neutrophil counts were less than 100/mm3 in 76.6% of all patients, and more than 3,000/mm3 in only 5.0%. In contrast to this result, in 66.7% of ATL patients and 37.5% of MM patients, septicemia occurred even when neutrophil counts were more than 3,000/mm3. Septicemia occurred in 28.2% of the total patients but died. The mortality rate in MM and AA patients (50.0% respectively) was higher than in other diseases. According to the mortality of each causative organisms, fungal septicemia had a terribly high mortality of 82.9% while other bacterial mortality was about 20%.