Using a commercially available computer-based analysis of ventilation and esophageal pressure (PeDs), we studied 11 healthy term neonates with serial pulmonary mechanics measurements during the first 3 days of life to determine the intrasubject variability of repeated measurements. Three consecutive pulmonary function tests were obtained before and after repositioning of the esophageal catheter, for a total of six measurements per day in each subject. The daily coefficient of variation for these 11 subjects ranged from 6% to 32% for respiratory rate; 6% to 27% for tidal volume; 3% to 28% for specific dynamic compliance, and 11% to 69% for pulmonary resistance. Repositioning the esophageal catheter did not produce significant differences in measurements of pulmonary mechanics (p greater than 0.05). We conclude that within a given subject, the maximum variability (mean + 2 SD) was 28% for specific dynamic compliance and 56% for pulmonary resistance. These intrasubject variability limits are important when one is interpreting pulmonary mechanics measurements in neonates before and after specific treatment, such as bronchodilators, diuretics, surfactant, or steroids.