Extensively drug resistant (XDR) Acinetobacter baumannii infections are increasing. Knowledge of risk factors can help to prevent these infections. methods. We designed a 1: 1: 1 case-case-control study to identify risk factors for XDR A. baumannii bacteremia in Singapore and Thailand. Case group 1 was defined as having infection due to XDR A. baumannii, and case group 2 was defined as having infection due to non-XDR A. baumannii. The control group comprised patients with blood cultures obtained to determine possible infection. results. There were 93 patients in each group. Pitt bacteremia score (adjusted odds ratio [aOR], 2.570 [95% confidence interval (CI), 1.528-4.322]), central venous catheters (CVCs; aOR, 12.644 [95% CI, 2.143-74.620]), use of carbapenems (aOR, 54.391 [95% CI, 3.869- 764.674]), and piperacillin-tazobactam (aOR, 55.035 [95% CI, 4.803-630.613]) were independently associated with XDR A. baumannii bacteremia. In case group 2, Pitt bacteremia score (aOR, 1.667 [95% CI, 1.265-2.196]) and third-generation cephalosporins (aOR, 2.965 [95% CI, 1.224-7.182]) were independently associated with non-XDR A. baumannii bacteremia. Concurrent infections (aOR, 3.527 [95% CI, 1.479-8.411]), cancer (aOR, 3.172 [95% CI, 1.135-8.865]), and respiratory source (aOR, 2.690 [95% CI, 1.160-6.239]) were associated with an increased risk of 30-day mortality. Survivors received more active empirical therapy (16.7% vs 9.6%; P=157), had fewer cases of XDR bacteremia (45.8% vs 52.6%; P=.452), and received higher median definitive polymyxin B doses (840,000 units vs 700,000 units; P=.339) conclusions. Use of CVC and broad spectrum antibiotics were unique risk factors of XDR A. baumannii bacteremia. Effective antimicrobial stewardship together with use of a CVC bundle may reduce the incidence of these infections. Risk factors of acquisition and mortality may help identify patients for early initiation of polymyxin B therapy. © 2013 by The Society for Healthcare Epidemiology of America. All rights reserved.