Nosocomial pneumonia is frequent and is associated with high mortality in intubated mechanically ventilated patients. To determine whether there is a significant relationship between subclinical aspiration from nasogastric feeding and development of nosocomial pneumonia, we studied 24 ventilated patients who received nasogastric feeding. Endotracheal aspirates were tested twice daily for the presence of glucose using a glucose oxidase reagent strip. Subclinical aspiration was defined by the presence of glucose in nonbloody endotracheal aspirates. Nosocomial pneumonia was defined by the presence of all of the following conditions: (1) new or worsening infiltrate on chest roentgenogram consistent with pneumonia, (2) temperature greater than 38 degrees C and/or white blood cell count greater than 10,000/mm3 with 10% or more band forms, (3) culture of a new organism in the sputum, and (4) purulent tracheobronchial secretions. Twelve patients had no positive glucose tests of their endotracheal aspirates during their intensive care unit stay, one of whom developed nosocomial pneumonia. Twelve patients had endotracheal aspirates that were positive for glucose on 1 to 5 occasions. Of these patients, seven had bloody glucose-positive aspirates, four of whom developed clinical pneumonia. The remaining five patients had nonbloody glucose-positive endotracheal aspirates and all developed nosocomial pneumonia. There was a significantly greater incidence of pneumonia among patients who had nonbloody glucose-positive aspirates than among patients without glucose-positive aspirates (p less than 0.001, Fisher's exact test). We conclude that subclinical aspiration of nasogastric feeding, as detected by nonbloody glucose-positive endotracheal aspiration, is associated with the development of nosocomial pneumonia.