We measured the anteroposterior and lateral diameter of the rib cage (RC), the anteroposterior diameter of the abdomen (ABD) and the cranio caudal abdominal dimension during breathing in supine posture, in order to analyze the shape of the chest wall in both awake and sleep conditions. By comparing the active breath holding and relaxation curves, it appeared that during activity of the respiratory muscles the RC is both expanded and distorted at the highest volumes and mainly distorted at the lowest volumes. During sleep the abdominal protrusion is smaller and the lateral sides of the rib cage expand more than during wakefulness. This might explain the higher rib cage motion during sleep found by some authors who measured the rib cage circumference and not confirmed by others who measured only its anteroposterior diameter. The motion of the rib cage is similar during sleep and wakefulness, its lateral parts leading the anteroposterior ones, showing that the pattern of motion of the rib cage is not affected by the different activation of the respiratory muscles. The possibility of a distorsion within the front part of the rib cage has been also discussed.