Intensive care units (ICU) are the areas with the highest rates of nosocomial infections, emergence and spread of resistant pathogens. In this study, a total of 1041 isolates which were recovered from various clinical specimens obtained weekly form the ICU patients in our hospital from January 2003 to January 2006 were evaluated. Specimen sources included tracheal aspirate (52.8 %), urine (21.9%), blood (20.4%) and 4.9% other various specimens. Of the total number of isolates, 77% were Gram negative bacteria and the remaining 23% were Gram positive bacteria. Among them, Pseudomonas spp was the most commonly isolated microorganism (26.8%), followed by Staphylococcus aureus and Escherichia coli, 19.9% and 14.9 % respectively. The most active antibiotics against Gram negative bacteria were imipenem (with 66.9% susceptibility) and meropenem (66.9%), followed by cefoperazone-sulbactam (50.4%), piperacillin-tazobactam (49.1%), amikacin (48.5%) and ciprofloxacin (46.7%). The expanded-spectrum ß lactamase (ESBL) production rate in Gram negative bacteria was 13.3%. Among Gram negative pathogens, Klebsiella spp and E.coli had the highest rates of ESBL production, 27.9% and 16.8% respectively. When the difference of susceptibility rates between years was investigated, only the decrease in ofloxacin and ciprofloxacin susceptibility was found to be statistically significant (both p<0.005). While 75% of S.aureus isolates had methicillin resistance, no glycopeptide resistance was detected in S.aureus and enterococci. In conclusion, the ICU flora in our hospital is mostly composed of Gram negative microorganisms. A unique spectrum of microorganisms with different resistance patterns cause infections in each ICU. Therefore, surveillance of the microbiologic profile and the antibiotic susceptibility patterns in every unit is important for decision on antimicrobial therapy and management of infection control.