Neutrophil counts were studied in 62 preterm infants receiving mechanical ventilation for neonatal respiratory distress syndrome (NRDS). Exploratory analysis indicated that the severity of NRDS, as demonstrated by fractional inspired oxygen (FiO2), mean airway pressure (MAP), arterial-alveolar PO2 ratio (a/APO2) and oxygenation index (OI), was correlated with percentage change of neutrophil counts during the first 5 days of life. Further analysis demonstrated that infants with NRDS who subsequently developed chronic lung disease (CLD) (n = 21) had statistically significant differences in variation of neutrophil counts when compared with the remainder (n = 41) without CLD (-35.0% +/- 4.3 vs. -16.9% +/- 5.8, p < 0.02). It is concluded that significant variations in neutrophil counts during the first 5 days of life may be found in infants with NRDS who subsequently develop CLD and that these changes may have predictive value regarding the development of CLD.