We performed susceptibility testing using the microdilution method to determine the in-vitro activity of tigecycline against 393 Acinetobacter baumannii clinical isolates collected in 2006 from 19 hospitals in Taiwan. Significant proportions of the isolates were resistant to imipenem (44%), ciprofloxacin (75%), amikacin (69%), sulbactam (34%) and all four antibiotics (22%), and susceptibility to tigecycline among these different resistant phenotypes of A. baumannii varied from 71% to 82%. The minimum inhibitory concentration (MIC) of tigecycline ranged from 0.6 to 16 microg/mL (MIC(50) 2 microg/mL; MIC(90) 4 microg/mL). The cumulative curve of tigecycline MICs showed that when the MIC cut-offs were set at 2 microg/mL and 4 microg/mL, 80.9% and 93.1% of the isolates were susceptible, respectively. As tigecycline will be used in the future for infections caused by multidrug-resistant A. baumannii because of limited antibiotic choice, and as resistance to tigecycline in A. baumannii isolates may develop following antibiotic exposure, continuous monitoring of the susceptibility of A. baumannii isolates to tigecycline is warranted.