Continuous positive airway pressure (CPAP) is widely advocated for the treatment of respiratory complications. However the effects of CPAP on the respiratory function of tetraplegic patients have not yet been investigated. The purpose of this study was to examine the effects of breathing with different levels of CPAP on the relationship between closing volume (CV) and functional residual capacity (FRC) in ten recently injured, but otherwise healthy tetraplegic patients with lesions between the fourth and eighth cervical segments. Lung volumes were measured before, during and after 32 min of zero end-expiratory pressure and 5 and 10 cm H2O of CPAP. FRC was measured by the open-circuit nitrogen washout method and CV was measured by the single breath nitrogen washout method. FRC was unaffected by zero end-expiratory pressure, but both 5 cm H2O and 10 cm H2O of CPAP caused significant increases in FRC. FRC returned to pre-CPAP values by the first minute after removal of 5 and 10 cm H2O of CPAP. We were unable to measure CVs in any subjects. It was concluded that 5 and 10 cm H2O of CPAP increase FRC in healthy tetraplegic individuals, but that these increases are rapidly lost with the subsequent removal of CPAP. These results suggest that CPAP may have a role in the treatment and prevention of respiratory complications in tetraplegics.