The retrograde microorganismal colonization in the pharynx from stomach may cause the nosocomial pneumonia and that may be more likely when the gastric pH is relatively high. We tried to find out the relationships between the gastric pH and the incidence of nosocomial pneumonia with twenty patients intubated for longer than 48 hours at ICU. We achieved following results: 1) The incidence of the nosocomial pneumonia was twenty percent. 2) All the patients developed nosocomial pneumonia showed the gastric pH above 4.0. 3) In the patients intubated for longer than 5 days, the incidence of nosocomial pneumonia was 33.3% in contrast to 9.0% for less than 5 days. 4) With the sputum culture, the incidence of colonization was higher in the patients with gastric pH above 4.0 than that in the patients with gastric pH below 4.0(84.6% vs 58.1%). 5) With regard to the duration of intubation, the incidence of colonization was higher in the patients intubated for longer than 4 days than that in the patients intubated for less than 4 days(90% vs 50%). 6) The most common pathognomic organisms were astreptoccus and Pseudomonas aeroginosa. It is conculded that the nosocmial pneumonia might develop more frequently in the patients with gastric pH above 4.0 than in the patients with gastric pH below establishment of the relationship between the treatment of the stress ulcer and the nosocomial pneumonia.