Seventy-four cases of pneumonia admitted to the Medical Intensive Care Unit of Chulalongkorn Hospital from January 1986 to December 1988 were analyzed. There were 52 males and 22 females with 46, 26 and 2 cases of community-, hospital- and combined-acquired pneumonia, respectively; the average age on admission was 58 +/- 18.1 years. The majority of the patients had abnormal host or underlying pulmonary diseases. Most of the cases with community-acquired pneumonia were admitted to the ICU because of respiratory failure (85%) and most of the cases with hospital-acquired pneumonia had ventilator-associated pneumonia (71.4%). The dominating pathogens in community- and hospital-acquired pneumonia were Gram-positive and Gram-negative organisms, respectively. There were no significant differences in initial clinical manifestations, laboratory data, complications, between the two groups. However, the duration on antibiotic, respirator, leukocytosis and stay in hospital were significantly longer in hospital acquired than community acquired pneumonias. Overall mortality was 63.5 per cent, the causes of which were progressive or uncontrolled pneumonia and mechanical ventilation complications in 37.8 and 25.7 per cent of the cases, respectively. High-risk factors of the non-surviving groups included pneumonia on assisted ventilation, mechanical ventilation complications, shock and the presence of Gram-negative bacilli. Our study indicates that the patients required assisted ventilation because of the severity of the underlying pulmonary conditions rather than the extent of the pneumonia itself. Gram-negative pneumonia predisposed the patients requiring mechanical ventilation to develop complications. The duration on mechanical ventilation, ICU admission and hospitalization were 13.84, 13.21 and 29.08 days, respectively.