Abstract: The study was conducted in patients in department of neurology and neural surgery of three teaching hopitals requiring mechanical ventilation or tracheostomy,the secretion of lower respiratory tract was collected by protected specimen brush(PSB)and the specimens were carried out for quantitative culture of bacteria.At the same time,the bacterial samples of gastric aspirates were collected to culture and the pH value of gastric aspirates was determined using a digital acid-base analyzer.All patients were monitored for development of nosocomial pneumonia every 2 days.The gene typings of the bacterial from lower respiratory tract secretion and gastric aspirates were take by plasmial profile analysis,restriction andonuclease fingerprint analysis and genome macrorestriction-pulsed field gel electrophoresis(GM-PFGE)in the same phenotype bacteria In 11(22%) case of nosocomial pneumonia,the identical organisms were isolated first in the gastric samples and then one to two days later in tracheal secretion.They were very homolgous,exhibiting the same fingerpriting in plasmid profile,endonuclease fingerpint and GM-PFGE.Two days after being perfused into the stomach through masogastric tubes,Methylent blue was found in the secretion of throat and lower respiratory tract.The bacterial counts in gastric aspirates increased significantly with gastric pH.Conclusion The retrograde route of transmission is confirmed to be existed and it is one of the risk factors in nosocomial pneumonia.The occurrence of nosocomial pneumonia is significantly associated with rising of gastric pH.