The safety and efficacy of meropenem and imipenem/cilastatin were compared in the treatment of hospitalized patients with complicated urinary tract infections (UTIs) in a prospective, multicenter, open, parallel-group trial. Patients were randomized to receive 500-mg intravenous doses of either meropenem every 8 hours (n = 116) or imipenem/cilastatin every 6 hours (n = 119). Data from 95 patients given meropenem and 82 patients given imipenem/cilastatin were included in the evaluation of efficacy. Meropenem produced satisfactory clinical and bacteriologic responses in 99% and 90% of cases, respectively. These results were similar to those obtained with imipenem/cilastatin, which produced clinical improvement in 99% of cases and bacteriologic improvement in 81%. Two patients given meropenem and five patients given imipenem/cilastatin developed superinfections. The rate of relapse was similar in the two groups. Patients who received meropenem had fewer drug-related adverse reactions than did recipients of imipenem/cilastatin (8% vs. 19%). The results of this study demonstrate that meropenem is a safe and effective alternative to imipenem/cilastatin in the treatment of hospitalized patients with complicated UTIs.