Imipenem/cilastatin, which combines a broad-spectrum antibiotic derived from thienamycin with a specific enzyme inhibitor, was administered in dosages of 1 to 4 gm/day to 717 patients in a multicenter noncomparative trial. Ninety-nine percent of the bacterial pathogens tested were susceptible to imipenem, and 86% were eradicated. Clinical outcome was favorable in 85% or more of the cases when assessed according to the site of infection, and 92% of the cases responded to treatment overall. Development of resistance was rare except for Pseudomonas aeruginosa, which became resistant in 19% of the patients infected with that organism. More than half the patients with resistant P aeruginosa had a favorable clinical outcome, however. Superinfection occurred in approximately 4% of all patients. The adverse clinical experiences occurring most frequently were related to gastrointestinal function (nausea, vomiting, and diarrhea). In general, the safety profile of imipenem/cilastatin was similar to that of other beta-lactam antibiotics.