The authors measured antidiuretic hormone (ADH) excretion and renal and cardiovascular function in eight patients with flail chest during four ventilatory conditions: continuous positive-pressure ventilation (CPPV), intermittent positive pressure ventilation (IPPV), spontaneous breathing with continuous positive airway pressure (CPAP) and spontaneous breathing (SB). Each condition was maintained for six to eight days. ADH excretion was significantly greater during CPPV (181 +/- 14 ng/24 h, mean +/- SE) than during IPPV (86 +/- 10), CPAP (181 +/- 14 ng/24 h, mean +/- 6). Free-water clearance was more negative during mechanical ventilation than during CPAP and SB, resulting in significant fluid retention during CPPV and IPPV (440 +/- 88 and 547 +/- 70 ml/day) and a negative water balance during SB (-154 +/- 80 ml/day). Arterial and central venous pressures and cardiac output (measured in four patients) did not change significantly among ventilatory conditions. It is concluded that ADH excretion is increased during CPPV, and this could represent one possible mechanism of water retention with this type of ventilation.