This study was conducted to determine the effect of the local environment within the host on the ability ofP. aeruginosato produce different extracellular virulence factors (elastase, phospholipase C, toxin A, and exoenzyme S). A total of 105P. aeruginosaisolates was obtained from patients with tracheal, urinary tract, and wound infections. Quantitative analysis of the virulence factors was done by growing the isolatesin vitroin different defined media. Single colonies of each isolate were inoculated from the primary isolation plates into the defined medium. All four virulence factors were produced by most isolates. However, depending on the location of their isolation, the isolates varied in the level of virulence factors they produced. High levels of elastase and phospholipase C were produced by most isolates obtained from trachea, urinary tract, and wounds. A significantly higher level of toxin A was produced by wound isolates, while a significantly higher level of exoenzyme S was produced by wound and urinary tract isolates. SomeP. aeruginosastrains were frequently isolated from the same site of infection (persistent infection isolates). Comparative analysis of virulence factors produced by these isolates showed that, regardless of the isolation site, subsequent isolates produced higher levels of exoenzyme S. These results suggest that: (1) elastase, phospholipase C, toxin A, and exoenzyme S are produced byP. aeruginosaisolates from different sites of infection; (2) the production of higher levels of elastase and phospholipase C is important in all types of infections, while the production of toxin A and exoenzyme S is important in wound infection; (3) persistent infection withP. aeruginosamay enhance exoenzyme S production.