Rates of antibiotic resistance in Pseudomonas aeruginosa isolates from intensive care unit (ICU) patients are expected to be dependent on the selection of resistance mutations during therapy, the availability of exogenous resistance determinants and their dissemination potential, and the efficiency of transmission of the resistant strains. The relative contributions of these three factors were studied in an ICU with no apparent outbreak in 216 sequential P. aeruginosa isolates recovered from 102 patients between September 2002 and November 2003. Analysis of pulsed-field gel electrophoresis patterns revealed the presence of 82 different clones. Thus, the dissemination of particular resistant clones had a minimal effect on the relatively high overall resistance frequencies found for imipenem (32%), cefepime (25%), ceftazidime (24%), meropenem (22%), ciprofloxacin (18%) and tobramycin (2%). Rates of primary resistance were relatively low, and resistance development during treatment (secondary resistance) was the main factor contributing to the overall high resistance rates. In ICU settings with a low prevalence of epidemic resistant strains, the main strategy for resistance control should focus on the design of targeted regimens to avoid the development of resistance.