In the present study, the prevalence of aerobic microorganisms, anaerobic microorganisms, fungi and Legionella species were estimated among critically ill patients suffering from pneumonia. The study was conducted over a period of four months from February till May, 2003 on fifty nine critically ill patients suffering from lower respiratory tract infection (pneumonia) in the CCU at Kasr El-Aini Hospital. Samples as sputum and endotracheal aspirate were taken for preparation of direct smears, culture aerobically on blood agar, chocolate agar, MacConkey’s medium, BCYE medium and Sabouraud’s medium and anaerobically using gas pack BBL system. Then identification was done by colony morphology, gram stained smear, specific tests for some organisms, and by the Sensititre system. The total number of patients admitted to CCU was 969. Fifty nine (6%) patients were identified as lower respiratory tract infection (pneumonia); thirty three cases (56%) were diagnosed as CAP, ten (30.3%) of them were previously hospitalized. The remaining twenty six cases (44%) were diagnosed as HAP. The most prevalent organisms isolated from patients with HAP were EGNB (58%) followed by NF-GNB (54%) (Pseudomonas aeruginosa, other Pseudomonas species, Stenotrophomonas maltophilia, and Acinetobacter species). While in CAP patients the commonest isolates were gram-positive cocci and fungal species (33%) followed by EGNB (27%). Infection with a single aetiological organism (monomicrobial) was usually found among CAP group and in mechanically ventilated HAP patients. MRSA was the predominant aetiologic pathogen among HAP group especially in patients suffering from CNS disorders. Gram-negative bacilli mainly Klebsiella pneumoniae played a major role among patients with HAP specially those suffering from cardiac lesions and diabetes mellitus. Legionella pneumophila was found to play a non significant role in both CAP and HAP groups. No growth of pathogenic organism was found in (18%) of CAP patients which makes the aetiological diagnosis of pneumonia incomplete among this group. Increased incidence of pneumonia occurred with increased age. Hospital-acquired pneumonia developed mainly in patients under mechanical ventilation and with increased duration of hospitalization. The use of antacids is an important risk factor for the development of hospital acquired pneumonia. It is recommended that duration of hospitalization should be shortened. Antibiotic use should be restricted for patients in community and hospital and culture should be requested after clinical diagnosis of respiratory tract infection in order to be able to diagnose true aetiological agent. The use of simple infection control measures and proper disinfection of respiratory therapy equipment in patients requiring mechanical ventilation could prevent the development of hospital-acquired pneumonia.