In vitro activity of tigecycline against 148 strains of Acinetobacter baumannii isolated from different patients hospitalized at Siriraj Hospital, Bangkok, Thailand during 2002 to 2005 was conducted. These isolates were resistant to beta-lactams, aminoglycosides and fluoroquinolones. In vitro susceptibilities were determined by Kirby-Bauer disk diffusion, E-test and broth microdilution methods. The MIC50 and MIC90 values of tigecycline against A. baumannii determined by the broth microdilution method were 0.5 and 1 mg/L respectively. The MICs of tigecycline determined by E-test were 4-fold higher than those from the broth microdilution method. An inhibition zone of > or = 13 mm was well correlated with a tigecycline MIC of < or = 2 mg/L and had a sensitivity of 99% and a specificity of 100%. The study results indicated that 97.3% of MDR A. baumannii strains isolated from the patients hospitalized at Siriraj Hospital were susceptible to tigecycline. Tigecycline may prove to be an important antibiotic for treatment of multidrug-resistant A. baumannii infections in Thailand in the near future.