The efficacy and safety of intravenous (i.v.) ertapenem (1 g once a day) with the option to switch to an oral agent for treatment of adults with complicated urinary tract infections (UTIs) were compared with that of i.v. ceftriaxone (1 g daily) with the same oral switch option in a multicenter, double-blind, prospective, randomized study. At entry, 592 patients were assigned to one of two strata: acute pyelonephritis or other complicated UTI without acute pyelonephritis. After a minimum of 3 days, patients could be switched to an oral antimicrobial agent. A total of 159 patients in the ertapenem group and 171 patients in the ceftriaxone group were microbiologically evaluable. Approximately 95% of patients in each treatment group were switched to oral therapy. The most common pathogens were Escherichia coli and Klebsiella pneumoniae. At the primary efficacy endpoint 5 to 9 days after treatment, 91.8% of patients who received ertapenem and 93.0% of those who received ceftriaxone had a favorable microbiological response (95% confidence interval for the difference, adjusting for strata, -7.6 to 5.1%), indicating that outcomes in the two treatment groups were equivalent. Microbiological success rates for the two treatment groups were similar when compared by stratum and also by severity of infection. The frequency and severity of drug-related adverse events were generally similar in both treatment groups. In this study, ertapenem was as effective as ceftriaxone for the initial treatment of complicated UTIs in adults, was generally well tolerated, and had a similar overall safety profile.