Doripenem is an antibiotic medication not yet approved by the US FDA. This is a phase 3, multi-center, randomized, prospective, double-blind study of doripenem versus comparator in the treatment of complicated lower urinary tract infections (UTI) or pyelonephritis in adults. After the screening, patients are randomized to receive either doripenem or comparator. Patients are hospitalized, but subsequently, patients may be treated as outpatient or through home-based therapy at the investigators' discretion. Conventional laboratory data are collected from all patients at specified times throughout the study. The primary endpoint is microbiological response measured at test of cure visit. The patients receive either doripenum or comparator; duration of therapy is 10 days.