Background: The Study for Monitoring Antimicrobial Resistance Trends (SMART) is examining aerobic and facultatively anaerobic gram-negative bacilli (GNB) isolated from intra-abdominal infections. This report summarizes the 2005 annual data. Methods: During 2005, 76 medical centers in 31 countries in five regions collected intra-abdominal GNB for antimicrobial susceptibility testing using broth microdilution according to the Clinical and Laboratory Standards Institute guidelines. Results: A total of 5,476 unique aerobic and facultatively anaerobic GNB were isolated. Enterobacteriaceae accounted for 86% (4,711) of the total isolates. Among the 12 antimicrobial agents tested, the carbapenems and amikacin were the most reliably active against the Enterobacteriaceae, whereas ampicillin/sulbactam most often was the least active. Escherichia coli was the species most commonly isolated, at 48% (2,654). Extended-spectrum beta-lactamases (ESBLs) were detected phenotypically in 12% (325/2,329) of E. coli and 18% (151/856) of Klebsiella spp. In general, ESBL producers demonstrated lower susceptibility to the majority of the antibiotics than the non-producers; however, ESBL producers usually were susceptible to the carbapenems tested. Conclusions: In 2005, antibiotic resistance continued to be a problem among GNB isolated from intra-abdominal infections, with the highest resistance rates observed in the Asia/Pacific region. Imipenem-cilastatin, ertapenem, and amikacin were the agents most consistently active in vitro against the Enterobacteriaceae isolated.