Imipenem (N-formimidoyl thienamycin) is a new carbapenem beta-lactam antibiotic with a broad antibacterial spectrum. Forty-five patients were treated with either 500 or 1,000 mg of imipenem/cilastatin four times daily, the duration varying according to clinical response. The diagnoses were urinary tract infection, 10 patients; septicemia, six; intraabdominal sepsis, six; pneumonia, six (two cases of Legionnaires' disease); skin and soft tissue infection, four; and other diagnoses, 13. Of the 32 clinically assessable patients, 17 were cured, nine improved, three died, and three were withdrawn from the trial. Of 21 patients who were microbiologically assessable, 13 were cured. In six cases of complicated urinary tract infection, the organism--which had been eradicated from the urine during treatment--reappeared after completion of antibiotic therapy. Two patients developed adverse clinical reactions that were thought to be drug-related (drug-induced fever and nausea plus vomiting, respectively). Both patients had mildly abnormal results in liver function tests, and one developed a positive direct Coombs' test. Fifty-seven percent of the patients developed some degree of phlebitis, which was moderate to severe in 19%. In this study imipenem/cilastatin proved to be a highly effective agent for the treatment of a variety of serious bacterial infections.