CONCLUSION: A case-control study in medical and surgical patients in Siriraj Hospital in 2005 revealed risk factors for AB nosocomial infection. Prolonged admission of more than 2 weeks, use of devices, and prior treatment with certain antimicrobials were found to be significant risk factors for the infection. To reduce the infection, strict infection control measures must be applied to the patients with these risk factors. Education to medical personnel and enforcement of infection control practices are all needed to reduce antimicrobial resistant bacterial nosocomial infection.