Ventilators can impose resistive and elastic loads during subject-initiated and spontaneous breaths. Such loads might worsen the chest wall distortion that is characteristic of patients with flail chest. We have tested this expectation in nine patients with flail chest and four normal subjects. All subjects breathed for 3 to 5 min on each of the following modes: assist control, intermittent mandatory ventilation (IMV), continuous positive airway pressure 5 to 10 cm H2O by demand valve and by a high flow system (CPAP-HF), and spontaneously (T-piece). Pressure at the airway opening was evaluated as a measure of ventilator loading, and magnetometric displacements of the major chest wall dimensions were evaluated to assess chest wall distortion. In contrast to the normal volunteers, patients with flail chest displayed chest wall distortion during active inspirations. The patterns of distortion were variable among patients. The degree of distortion varied among ventilator modes; generally, there was a greater degree of chest wall distortion in breaths with greater loading. For example, distortion was greater during the spontaneous breaths taken on the IMV-mode than during spontaneous breaths taken on the T-piece. The CPAP-HF mode resulted in the least distortion, reversing chest wall distortion in five patients, improving it in two, and not changing the distortion in the remaining two. The improvements may be related to positive pleural pressures and to the minimal ventilator-imposed load of the high gas flow system. The distortion imposed by ventilators increases the work of breathing in these patients and may thus contribute to difficulty in weaning.