Pseudomonas aeruginosa is an important nosocomial pathogen. It causes high morbidity and mortality. The incidence of imipenem-resistance is increasing among Gram negative bacilli, particularly P. aeruginosa, often associated with resistance to other antipseudomonal drugs. The aim of this study is to assess potential risk factors for nosocomial infections of imipenem-resitant P. aeruginosa (IRPA), and to obtain base line information on the epidemiology and susceptibility pattern of this important pathogen. In a retrospective study of patients with nosocomial P. aeruginosa infection from July 2008 to August 2010 in a tertiary care hospital in Riyadh, Saudi Arabia, the imipenem susceptibility of 381 nosocomial P. aeruginosa isolates was evaluated by disk diffusion method and confirmed by determining minimum inhibitory concentration (MIC) using E test. Antimicrobial susceptibility testing for other antibiotics was performed. The features of the patients with IRPA infections were compared to those with imipenem-sensitive P. aeruginosa (ISPA) infections. During the study period, IRPA was isolated from 60 (15.7%) of patients, and the ISPA was isolated from 321 (84.3%) patients. IRPA were most frequently isolated from urine (23.3%) cultures (p=0.01), whereas ISPA were most frequently isolated from swab (57.6%) cultures (p=0.04). Risk factors that were significantly associated with IRPA included: surgery (p=0.001), diabetes mellitus (p=0.04), and trauma (p=0.01). Overall, the highest P. aeruginosa rate (33%) was obtained in the medical ward followed by ICU (29%), surgical ward (28.3%), oncology (5.5%), burn unit (3%), and renal dialysis unit (2.4%). ICU patients accounted for 21(35%) of IRPA infected patients. All tested antipseudomonal agents, demonstrated higher resistance rates among IRPA isolates than ISPA isolates (p=0.001). Pipracillin/tazobactam was the most active agent for both groups followed by amikacin and ceftazidime. No significant difference in the mortality rate between the imipenem-resistant and susceptible groups. Isolates of P. aeruginosa that are resistant to all, or almost all, available antibiotics "panresistant" are now prevalent worldwide. Our study showed that imipenem-resistance in this clinical pathogen is an emerging problem. Periodical surveillance studies are therefore essential to determine the current susceptibility patterns of P. aeruginosa to different antimicrobial agents, and to identify risk factors for acquisition of carbapenem-resistant strains.