The pressure-time product (PTP) and work of breathing (W) were measured in 19 intubated patients during weaning from mechanical ventilation after aortocoronary bypass surgery. The patients were supported by two different ventilatory modes: biphasic continuous positive airway pressure (Biphasic CPAP; a ventilatory mode that permits spontaneous breathing at two different levels of airway pressure during inspiration and during expiration) and pressure-support ventilation (denominated as assisted spontaneous breathing, ASB). Our aim was to compare the energy cost of breathing imposed by both modes. The PTP was obtained by integrating the area enclosed by the esophageal and chest-wall static recoil pressure curves. The W performed by the patient (Wp) was calculated using Campbell's diagram method, based on the esophageal pressure curve and normalized per liter of ventilation. Two comparable support levels, corresponding to a high and a low degree of mechanical support, were studied for each ventilator mode. The levels of support were generally higher than that necessary to overcome inspiratory resistance. We found significantly higher PTP values during biphasic CPAP than during ASB (p < 0.01), whereas the Wp was the same with biphasic CPAP and ASB at both support levels. We conclude that because of the higher PTP values measured during biphasic CPAP, this mode of ventilation appears to have been more exhausting for our patients. The discrepancy between PTP and Wp is probably due to the different mechanisms of support given by the two ventilatory modes and the greater patient effort is related to those respiratory cycles that are not helped by the ventilator and which represent nearly half of the breaths during biphasic.