Sixty-eight patients with severe infections associated with hematopoietic disorders were treated with imipenem/cilastatin sodium (IPM/CS) and the efficacy and safety of this drug were evaluated. 1. Fifty-nine patients were evaluable for the efficacy. Clinical efficacies were excellent in 10 patients, good in 24, fair in 11 and poor in 14, and the overall efficacy rate was 57.6%. 2. The clinical efficacy rates were 62% against septicemia and suspected septicemia, 40% against pneumonia and 100% against urinary tract infection (1 case). 3. The clinical efficacy rates when these patients were grouped according to numbers of neutrophils after treatment were: less than 100/mm3; 44.4%, 101-500/mm3; 58.3% and over 501/mm3; 60.5%. The efficacy rate was particularly excellent, 60.0%, for patients with neutrophil counts were less than 100/mm3 both before and after treatment. 4. Sixty-eight patients were evaluable for the safety. Side effects were observed in 5 patients and abnormal laboratory test values were observed in 5 patients.