Clinical studies of imipenem/cilastatin sodium (IPM/CS) were conducted in 40 pediatric patients. 29 out of the 40 patients were treated for infections and 11 for prophylaxis. The following results were obtained. 1. The response rate in 29 patients with infections was 79.3%. Among the 29 patients, 16 patients who presented with malignant diseases showed the response rate of 68.8%. The response rate was lower in patients with severe infections than in those with mild or moderate infections, and a lower response rate was associated with severe neutropenia. However, there were no differences in the response rates between patients who had previously been treated and those who had been untreated with other antibiotics. The response rate in 6 patients from whom causative organisms were isolated was 83.3% and that in the remaining 23 patients was 78.3%. 2. The response rate in 11 patients to whom IPM/CS was administered prophylactically was 63.6%. 3. As for side effects, a rash was observed in 1 patient and hematuria in another, and the abnormal laboratory test results observed were elevations of GOT and GPT in 1 patient. However, they were not clinically significant. From the above results, it appears that IPM/CS may be used as a drug of the first choice for the treatment of patients with severe infections in which the causative organisms are unknown, and for the prophylaxis of infection in patients with neutropenia.