A combination of aztreonam (AZT) and clindamycin (CLDM) was evaluated for severe infections associated with leukemia and related disorders. AZT is a monobactam antibiotic which has strong bactericidal effect against Gram-negative bacteria including Pseudomonas aeruginosa. CLDM which has strong antibacterial spectrum against Gram-positive and anaerobic bacteria, was chosen as a partner of AZT in order to complement the weak points of AZT. Fifty six patients were treated with the combination therapy. Among them, 51 patients were evaluable for the effectiveness. Five patients were excluded from the evaluation because 3 patients were subjected to additional therapy with other agents such as amikacin, miconazole and pulse therapy of a large dose of methylprednisolone, one had a fever episode apparently due to primary disease, and the remaining one was discontinued of the combination therapy of administration due to mild nausea after 3 days. Excellent responses were obtained in 17 (33.3%) patients and good responses in 20 (39.2%) patients, with a total rate of effectiveness of 72.5%. One patient with whom the combination therapy was stopped due to nausea, was included in the final evaluation of side effects. Side effects were observed in 2 patients of 50 and 40 years of ages (2/52, 3.8%), both of whom suffered with nausea. In the 50 years patient of acute myeloblastic leukemia, nausea occurred in a slight degree in 3 hours after the combined regimen was started. But, it disappeared during the continuation of the combination therapy. In the 40 years patient of acute myelomonocytic leukemia, mild nausea occurred after 3 day administration of the combined regimen. It disappeared soon after the cessation of the treatment. These results indicated that a combination of AZT and CLDM was an effective and safe regimen for the treatment of severe infections in patients with hematological disorders.