The efficacy and safety of quinupristin/dalfopristin for treatment of infections due to vancomycin-resistant Enterococcus faecium were evaluated in 24 hospitalized patients with documented infections (19 bacteremias, 5 localized infections) caused by vancomycin-resistant E. faecium that was susceptible to quinupristin/dalfopristin in vitro. Patients received iv quinupristin/dalfopristin at a dosage of either 7.5 mg/kg every 8 h or 5 mg/kg every 8 h. A favorable clinical response (cure or improvement) occurred in 19 (83%) of 23 evaluable patients; bacteriologic eradication occurred in 17 (74%) of 23 evaluable patients. A favorable clinical response was observed in 12 (80%) of 15 patients who were treated with 7.5 mg/kg of quinupristin/dalfopristin every 8 h and in 7 (88%) of 8 patients treated with 5 mg/kg of quinupristin/dalfopristin every 8 h. Two of four treatment failures were associated with a decrease in the in vitro susceptibility of vancomycin-resistant E. faecium to quinupristin/dalfopristin. Superinfections developed in 6 patients (26%), but only one was caused by Enterococcus faecalis that was resistant to quinupristin/dalfopristin. Myalgias and arthralgias were the only adverse events related to quinupristin/dalfopristin. These conditions occurred in 8 (33%) of 24 patients and were dose-related (8 cases in 16 patients treated with 7.5 mg/kg of quinupristin/dalfopristin every 8 h, no cases in 8 patients treated with 5 mg/kg every 8 h). Mortality associated with vancomycin-resistant E. faecium infection was 17% (4 of 23 patients), whereas mortality from other causes was 52% (12 of 23 patients). These results suggest that quinupristin/dalfopristin is effective as treatment for vancomycin-resistant E. faecium infections in critically ill patients with serious underlying conditions. Except for myalgias and arthralgias at higher dosages, the drug is well-tolerated.