Nosocomial pneumonia is a major cause of morbidity and mortality in hospitalized patients. The risk is especially high in the neonatal intensive care unit (NICU) particularly in infants with mechanically assisted ventilation. During the 5-year period of the study, 160 infants with problems including prematurity (60.6%), respiratory distress (55.6%) and birth asphyxia (45.0%) were admitted to the NICU. One hundred and thirty-three infants (83.1%) received mechanical ventilation. Nosocomial pneumonia was found in 65 infants (40.6%) or 88.3 cases per 1,000 ventilator-days. Low birth weight, prematurity, respiratory distress and hyperbilirubinemia were found more significantly in the pneumonia group. They underwent more manipulations such as the placement of an umbilical catheter and orogastric tube. Infants with pneumonia received mechanical ventilation at a higher percentage and for a longer period than those without pneumonia (96.9% vs 73.7%, odds ratio = 11.2, p = 0.000) with a mean duration of 11.7 and 3.5 days respectively (p = 0.000). The etiologic organisms recovered from hemoculture were Acinetobacter calcoaceticus var. anitratus 44.0 per cent, Enterobacter spp. 16.0 per cent, Klebsiella pneumoniae 16.0 per cent, coagulase-negative staphylococci 12.0 per cent. There was no concordance of the bacteriologic results in endotracheal aspirate culture and hemoculture in each infant. Leukocytosis and granulocytosis as well as blood gas values could not differentiate the presence of pneumonia. The mean hospital stay for the infants with pneumonia was longer (23.0 days vs 6.4 days, p = 0.000). Nosocomial pneumonia did not only prolong hospital stay but also contributed to mortality. Twenty-seven (41.5%) of the infants with pneumonia died, compared with 46 (48.4%) of the other group without pneumonia (p = 0.422). The risk of nosocomial pneumonia can be reduced by using infection control measures, including meticulous hand washing and gloving during respiratory manipulation, heat-treated water supply in a nebulizing unit of the ventilator and proper care of umbilical catheterization.