Antimicrobial susceptibility patterns of Gram negative pathogens (n: 699) isolated from nosocomial infections in Hacettepe University Adult Hospital included in MYSTIC programme between 2000-2004 were determined by Etest (AB BIODISK, Solna, Sweden) using CLSI criteria. Of the total Escherichia coli (n: 179), Klebsiella pneumoniae (n: 168), Pseudomonas aeruginosa (n: 179) and Acinetobacter baumannii (n: 173) isolates, 55% were from patients in inpatient clinics and 45% were from patients in intensive care units. The extended-spectrum beta-lactamase (ESBL) production rate of E. coli and K. pneumoniae isolates were 28% and 47%, respectively. All E. coli and K. pneumoniae isolates were found susceptible to carbapenems. The susceptibility of ESBL positive E. coli isolates to piperacillin+tazobactam was 76% and 65% to cefepime, 63% to ceftazidime, 56% to tobramycin, 24% to ciprofloxacin. For ESBL negative E. coli isolates, the susceptibility rates were found 95%, 92%, 84% and 58% for ceftazidime and tobramycin, cefepime, piperacillin+tazobactam and ciprofloxacin, respectively. The susceptibility rates of ESBL positive K. pneumoniae isolates to piperacillin+tazobactam was 55% and 51% to cefepime, 49% to ciprofloxacin, 45% to tobramycin, 42% to ceftazidime. For ESBL negative K. pneumoniae isolates, the susceptibility rates were determined as follows; 92% for cefepime, 87% for tobramycin, 70% for piperacillin+tazobactam, 62% for ceftazidime and 60% for ciprofloxacin. Of the P. aeruginosa isolates, 77% were multi-drug resistant with susceptibility rates to piperacillin+tazobactam, ciprofloxacin, meropenem and imipenem as 55%, 53%, 51% and 49%, respectively. The susceptibilities of P. aeruginosa isolates were 48% to ceftazidime, 41% to cefepime and 35% to tobramycin. Of the A.baumannii isolates, 67% were multi-drug resistant with higher susceptibility rates to meropenem (53%), imipenem (48%) and tobramycin (44%) compared to other antimicrobials cefepime (37%), ciprofloxacin (29%), piperacillin+tazobactam (26%) and ceftazidime (22%).