Using the envelope method, we allocated 125 patients with infections accompanied by hematopoietic disorders into two groups treated with imipenem/cilastatin sodium (IPM/CS) at a daily dose of 1 g/1 g b.i.d. (group BID) or 0.5 g/0.5 g q.i.d. (group QID), and obtained the following results. 1. In group BID, ANLL was observed in 25 patients; ALL in 6; and NHL in 12. In group QID, ANLL was observed in 27 patients; ALL in 7; and NHL in 13. 2. In group BID, efficacy rates were 54.5% (6/11) in sepsis, 63.0% (17/27) in fever of undetermined origin and 50.0% (4/8) in pneumonia, thus the overall efficacy was 61.8% (34/55). In group QID, efficacy rates were 66.7% (4/6) in sepsis, 76.0% (19/25) in fever of undetermined origin and 35.7% (5/14) in pneumonia, thus the over all was 61.1% (33/54). No significant difference in response rates were observed between the two groups. 3. Bacteriologically, 22 bacterial strains were isolated in group BID and 21 21 strains, in group QID. The eradication rates after treatment with IPM/CS was 100% in group BID and 66.7% in group QID. 4. Side effects were observed in 8 patients in group BID and 3 in group QID. Laboratory examination revealed abnormal values in 9 patients in group BID and 6 in group QID. However, all of the side effects disappeared after the suspension or discontinuation of IPM/CS. The efficacies of IPM/CS therapy for severe infections in patients with hematopoietic disease were similar between 1 g/1 g b.i.d. and 0.5 g/0.5 g q.i.d. groups.