Patients in intensive therapy units are especially susceptible to colonization and infection with Pseudomonas aeruginosa, those who develop pneumonia have a high mortality even when treated with apparently effective antibiotics. Pseudomonas aeruginosa is commonly found in the hospital environment, several reservoirs of infection have been identified and linked with outbreaks of infection. The aim of our work was to investigate the relative importance of sources (whether endogenous and exogenous), the prevalence of strain differences and the influence of host factors in colonization and infection. Fifty patients of different age and sex groups were included in our work, which was conducted in Intensive Care Unit in Kasr El-Ainy Hospital. Samples were taken from oropharynx, groin toe web region and urine, samples were also taken from the ventilators, suction apparatus, humidifiers, from detergent solutions, and from sinks and taps. Nurses hands' were sampled by contact impression on culture plates. Identification of isolates were made by culture on culture plates, and then by using API20E system, API Staph. system and by panel of biochemical reactions. The most common isolated organism from patients samples was Staph. aureus (30%), followed by Enterobacteriaceae (16%), and P. aeruginosa (12%), CNS represent (99.2%). There was positive correlation between the environmental isolated organisms and patient's isolated organisms, where Staph. aureus (30%) and P. aeruginosa (13.3%) were mainly isolated from ventilator equipments in the first cubicle, and Candida spp. (15%), were mainly isolated from sinks and taps in 2 second cubicle, there were no source of P. aeruginosa detected in nurses hands, yet Staph. aureus was mainly colonizing their right hands. Elimination of major reservoirs of P. aeruginosa and compliance with procedures to control cross infection remain essential if patients in hospitals are to escape colonization by this organism.