This investigation examined whether prolonged continuous positive airway pressure (CPAP) applied by face mask could sustain an increase in functional residual capacity (FRC). Before median sternotomy, nine adults performed multiple-breath nitrogen washout to determine FRC and spirometry. The day after operation, lung volumes were measured before and 10 min after the 4-h application of 7.5 cm H2O of CPAP with a mask. Mean FRC, forced vital capacity (FVC), forced expiratory volume in 1 sec (FEV1), and FEV1/FVC after CPAP were similar to pretreatment values. Although CPAP can restore FRC to preoperative values, and did increase FRC in these patients, FRC deteriorates within 10 min after CPAP is removed. Therefore, when a sustained increase in FRC is desired after median sternotomy, CPAP should be applied without interruption.