We studied effectiveness and safety of combination therapy of aztreonam (AZT) (2-6 g/day) and clindamycin (CLDM) (1,200-2,400 mg/day) on severe infections in neutropenic patients with hematological diseases. We administered AZT and CLDM to 250 consecutive patients and analyzed the results of a total of 212 cases for evaluation except 38 cases of exclusions and dropouts. Hematological malignancies constituted the dominating underlying diseases which represented 90.6% of all underlying diseases. The overall clinical efficacy of the combination therapy with AZT and CLDM was 65.6% and the following results were obtained for different categories: sepsis, 64.7%; suspected sepsis, 66.4%; and pneumonia 54.5%. Even severe patients with less than 100/microliters of neutrophil counts showed good responses with an efficacy rate at 57.4%. Side effects were observed in only 7 cases (2.8%). We concluded that the combination therapy with AZT and CLDM would be useful as empiric therapy against various infections in febrile neutropenic patients with hematological diseases.