Retractions of the lower ribcage (chest wall distortion [CWD]) during inspiration are frequently observed with moderate to severe respiratory disease in the infant. Laryngotracheobronchitis (LTB) results in a reversible partial airway obstruction with severe CWD. We wished to measure the motion of the chest wall during distortion to determine the changes in minute ventilation (VE) and to evaluate this clinical sign as a means of assessing disease severity. The respiratory inductance plethysmograph was used to determine the distortion of the lower chest wall, and distortion was correlated with VE, measured at the mouth, in six infants with severe LTB and ventilatory failure. As the conditions of these infants improved, the CWD decreased with decreasing transcutaneous carbon dioxide tension (tcPCO2), VE increased from 0.27 +/- 0.12 L.min-1 x kg-1 at a tcPCO2 of 64 mm Hg to 0.64 +/- 0.06 L.min-1 x kg-1 when the tcPCO2 had fallen to 28 mm Hg. Over the same change in tcPCO2, the tidal volume (VT) increased from 4.8 +/- 0.5 ml.kg-1 to 15.7 +/- 1.4 ml.kg-1. In the most severe disease state, the excursion of the chest wall (as an inductance) was -14 +/- 3 mV in severe obstruction, but increased to 75 mV +/- 4 mV with resolution of the illness. The timing and vector of movement of the abdomen and chest wall were expressed as a Lissajous figure, which is measured as a phase angle. The severity of the disease process, as determined by tcPCO2 was directly related to the phase relationship, and thus reflected both VE and VT. The severity of the CWD may be assessed rapidly by the use of Lissajous figures.