The clinical and bacteriological efficacy of imipenem/cilastatin (IPM/CS) was evaluated in 30 cases of serious infections associated with hematological malignancies. 1. Among 28 evaluable cases, excellent efficacy was obtained in 6 cases and good effectiveness in 10 cases, resulting in a high clinical efficacy rate (57.1%). The clinical effectiveness of IPM/CS was not dependent on neutrophil count in peripheral blood. A 53.8% efficacy rate was observed in 26 cases which had received pretreatment with other antibiotics. 2. Antibacterial activities of IPM/CS have so far been evaluated against organisms isolated in 20 of 28 cases: 2 strains of coagulase-negative Staphylococcus, 2 strains of methicillin-resistant Staphylococcus aureus, 2 strains of Enterococcus faecalis, 9 strains of Enterobacter cloacae, and 8 other strains. 3. Among 3 evaluable cases treated with IPM/CS alone, response was good in 1 case. Among 25 patients receiving IPM/CS in combination with an aminoglycoside or a penicillin, the efficacy rate was 60%. 4. Five patients had IPM/CS-related adverse events; nausea and vomiting in 2 cases, seizures in 1 case, small increases in GOT and GPT in 2 cases, and the appearance of casts in urine sediment in 1 case. These patients, however, tolerated the complete course of therapy with IPM/CS except the 2 cases with nausea and vomiting. These results indicate that chemotherapy with IPM/CS is effective for the treatment of severe infectious diseases accompanied by hematological disorders.