Axial movement of the right hemidiaphragm during tidal breathing was recorded using real-time ultrasonography in 46 healthy term infants. Displacement was 2.6 +/- 0.1, 3.6 +/- 0.2, and 4.5 +/- 0.2 mm (mean +/- SEM) for the anterior, middle, and posterior thirds, respectively. Diaphragmatic movement was significantly greater in the middle and posterior segments than in the anterior segment (P less than 0.0001). Excursion of the diaphragm was similar in sleeping and awake infants, and during quiet and active sleep, as identified by behavioral criteria. Diaphragmatic movement was also assessed in nine infants who required mechanical ventilation and pharmacologic paralysis because of respiratory disease. In these infants, axial movement of the right hemidiaphragm was less in the middle and posterior thirds (P less than 0.05 and P less than 0.01, respectively) than in spontaneously breathing infants, and posterior movement was not predominant. Normative data for axial diaphragmatic movement may be of clinical value in the assessment of defects of the diaphragm, rib cage, or abdomen in newborn infants and may allow further understanding of the direct effects of therapeutic interventions on the respiratory system in infancy.