Background: Multidrug-resistant pathogens have been on the rise during the last few years. Pseudomonas aeruginosa is commonly encountered in nosocomial infections with remarkable ability to develop antimicrobial resistance of which carbapenems are of great concern. Objectives: To explore the role of MexAB-OprM and MexXY-OprM efflux pumps overexpression as carbapenems resistance mechanisms among nosocomial P. aeruginosa isolates at both Menoufia and Kasr Al Ainy University Hospitals by phenotypic and molecular characterization methods. Methodology: A total of 120 P. aeruginosa isolates were collected from patients with hospital-acquired infections and subjected to antibiotic susceptibility testing by the Kirby-Bauer disk diffusion method. Carbapenems-resistant isolates were selected and investigated phenotypically for the contribution of MexAB-OprM and MexXY-OprM efflux pumps by both disk synergy and MIC reduction assays with cyanide-m-chlorophenyl hydrazone (CCCP) as an efflux pump inhibitor. Real time PCR assay verified the existence of mexA and mexX genes as regulators of MexAB-OprM and MexXY-OprM overexpression. Laboratory results were correlated with data regarding patients' clinical findings as well as risk factors. Results: Out of 120 P. aeruginosa isolates, 88 (73.3%) isolates were carbapenems-resistant of which 100% were MDR isolates. The highest resistance rate was for piperacillin and piperacillin/tazobactam (100% for each) and the lowest rate was seen against colistin (7.5%).The RT-PCR assay revealed that, 54/88 (61.3%) P. aeruginosa isolates harbored the target genes: 21 isolates (38.9%) were positive for mexA alone, 12 isolates (22.2%) were positive for mexX alone and 21 isolates (38.9%) showed co-existence of the two genes. In relation to PCR results, the sensitivity, specificity and accuracy of CCCP disk synergy test respectively were 46%, 94% and 64.8% while, those for MIC method were 88.9%, 55.9% and 76.1% respectively. Conclusion: Carbapenems resistance mediated by the overexpression of efflux pumps has also now emerged. Early recognition of this resistance mechanism to allow the use of alternative b-lactams is imperative.