Ninety-one Pseudomonas aeruginosa isolates recovered from cystic fibrosis and non-cystic fibrosis patients were evaluated regarding the ability to form biofilm and acyl-homoserine lactones production and for the presence of five quorum-sensing (QS) regulatory genes (lasI, lasR, rhlI, rhlR, and vfr). Most isolates (90.1 %) presented all five QS genes. Five isolates shown to be lasI/lasR-deficient were not able to produce biofilm in vitro. Moreover, one isolate harboring all five QS genes was also not able to form a biofilm. The function of rhlR gene may be compensated by the las QS system. However, in our study, all isolates which were deficient for the rhlR gene were also deficient for the lasI/lasR system. This may point to some hierarchy in QS regulation which may pose a potential for controlling biofilm infections due to P. aeruginosa.