The objective of this descriptive study was to study the antimicrobial utilization, type and susceptibility of pathogens, and drug therapy problems (DTPs) in nosocomial infected patients at Ratchaburi hospital from 1st August to 31st December 2002. Two hundred and seven patients were included in this study. Of which 58.5% were male and 41.5% were female with the mean age of 53.5±20.2 years. Culture tests were performed in 281 from the overall 305 nosocomial infections (92.1%). The top 5 pathogens found were P. aeruginosa (18.1%), A. baumannii (13.6%), E. coli (10.8%), K. pneumoniae (7.5%) and E. cloacae (6.8%). The susceptibility of the pathogens to the first three antimicrobials was: (1) P. aeruginosa was susceptible to amikacin, netilmicin and gentamicin at 93.0, 87.1, and 78.6%, respectively; (2) A. baumannii was susceptible to cefoperazone/sulbactam, ampicillin/sulbactam and imipenem at 72.7, 72.3, and 61.8%, respectively; (3) E. coli was susceptible to imipenem, amikacin and cefoxitin at 100.0, 97.6, and 78.0%, respectively; (4) K. pneumoniae was susceptible to imipenem, amikacin and norfloxacin at 92.9, 78.6, and 75.0%, respectively; and (5) E. cloacae was susceptible to imipenem, amikacin and co-trimoxazole at 92.3, 84.6, and 76.9%, respectively. Single antimicrobial (≥ 54.5%) was prescribed as empiric treatment more often than the combination of at least 2 antimicrobials in most infection sites, except for surgical site (37.3%). The most antimicrobial prescribed for empiric treatment of each infection site was: (1) lower respiratory tract infections were treated with the third generation cephalosporins as monotherapy (38.6%); (2) surgical site infections were treated with the penicillinase-resistant penicillins as monotherapy (15.7%); (3) pneumonia was treated with either the third or the fourth generation cephalosporins as monotherapy (45.8%); (4) urinary tract infections (UTI) were treated with the fluoroquinolones as monotherapy (47.3%); and (5) skin and soft tissue infections were treated with the penicillinase-resistant penicillins as monotherapy (34.7%). The empiric treatment of the UTI patients, the surgical site infected patients and the pneumonia patients were most corresponded to the results of the culture and susceptibility tests (38.7, 27.1, and 21.7%, respectively). The 163 DTPs were identified in 106 patients, most were from the drug interactions between antimicrobials and the concurrent administered drugs. However, the therapy outcome were not studied.