Growth of > or = 10(5) colonies of bacteria per milliliter obtained at bronchoscopy in children and adults correlates with bacterial pneumonia. To determine whether quantitative tracheal aspirate cultures aid in diagnosis of pneumonia in the neonatal intensive care unit setting, tracheal aspirates were obtained from 25 infants who had recently undergone endotracheal intubation; 15 of the infants had suspected pneumonia and 10 control infants had undergone intubation for suspected apnea of prematurity (4 infants) or elective surgery (6 infants). Studies also were performed to detect Mycoplasma, Ureaplasma, viruses, and Pneumocystis. Tracheal aspirates from 2 of 15 infants with suspected pneumonia grew > or = 10(5) bacteria, and 1 was positive for respiratory syncytial virus. These infants were considered to have pneumonia. In 12 infants whose tracheal aspirates grew < 10(5) bacteria, respiratory decompensation later was explained by other causes in 11 infants, and there was one false-negative culture. There were three false-positive tracheal aspirates in the control group. We conclude that tracheal aspirates of infants who have recently had an endotracheal tube placed may be useful for diagnosing pneumonia and for identifying the causative agent.