In order to assess incidence and magnitude of the "lower inflection point" of the chest wall, the sigmoidal equation was used in 36 consecutive patients intubated and mechanically ventilated with acute lung injury (ALI). They were 21 primary and 5 secondary ALI, 6 unilateral pneumonia and 4 cardiogenic pulmonary edema. The "lower inflection point" was estimated as the point of maximal compliance increase. The low constant flow inflation method and esophageal pressure were used to partitioning the volume-pressure curves into their chest wall and lung components on zero end-expiratory pressure. The sigmoidal equation had an excellent fit with coefficients of determination > 0.90 in all instances. The point of maximal compliance increase of the chest wall ranged from 0 to 8.3 cm H₂O (median 1 cm H₂O) with no difference between ALI groups. The chest wall significantly contributed to the lower inflection point of the respiratory system in 8 patients only. The occurrence of a significant contribution of the chest wall to the lower inflection point of the respiratory system is lower than anticipated. The sigmoidal equation is able to precisely determine the point of the maximal compliance increase of lung and chest wall.