Nosocomial pneumonia, which can develop in patients with prolonged intubation, may have in fatal consequences. The histamine type 2 blockers used for prevention of stress ulcers, increases gastric pH, and may increase the incidence of nosocomial pneumonia in patients with prolonged intubation by promoting bacterial overgrowth in the stomach and retrograde colonization in the trachea. We assessed the effect of histamine type 2 blockers on the incidence of nosocomial pneumonia in patients with prolonged intubation, who were admitted to the Intensive Care Unit, Young Dong Severance Hospital, Yonsei University College of Medicine from September 1992 to February 1993. The patients were divided into two groups Group 1 was the patients given histamine type 2 blockers and total parenteral nutrition, and Group 2 was the patients not given histamine type 2 blockers and with enteral feeding through the gastric tube. Gastric juice was aspirated for measurement of gastric pH. A culture of isolates was grown from endotracheal secretion and pharyngeal swab for identification of isolate cultured. Chest X-rays were taken for assess of pneumonic infiltrate, The results were as follows ;. 1. There were no difference of age, APACHE II scores at admission to ICU, duration of ICU stay and mortality between the two groups. 2. Gastric pH differed significantly between the two groups. In group 2, there was a significant difference between gastric pH before and after feeding. 3. The incidence of upper gastrointestinal bleeding did not differ between the two groups. 4. Colonization developed more frequently in the patients with enteral feeding. 5. The incidence of retrograde colonization was higher in patients with enteral feeding. 6. The incidence of nosocomial pneumonia were 30 and 40 percent, and there was no significant difference between the two groups. From our study, the incidence of nosocomial pneumonia was not different significantly between the two groups although gastric pH differed significantly.