Pseudomonas aeruginosa, a leading nosocomial pathogen, may become multidrug resistant (MDR). Its rate of occurrence, the individual risk factors among affected patients, and the clinical impact of infection are undetermined. A matched case-control study identified individual risk factors for having MDR P. aeruginosa, and a retrospective matched-cohort study examined clinical outcomes of such infections. Mex efflux pumps contribute to multidrug resistance in Pseudomonas aeruginosa. Evidencing their expression in clinical isolates would help in rationalizing antibiotic selection. We have developed a combined phenotypic and genotypic approach for the differential diagnosis of resistance mediated by four major transporters (MexAB-OprM, MexCD-OprJ, MexEF-OprN, MexXY-OprM). MIC measurements with reporter antibiotics [carbenicillin (MexAB-OprM); erythromycin (MexCD-OprJ); norfloxacin and imipenem (MexEF-OprN); gentamicin (MexXY-OprM)] with and without Phe-Argβ- naphthylamide. Genotypic detection was made by semi-quantitative reverse transcription PCR.