We have examined the role of age on the continuous positive airway pressure (CPAP) levels required to treat two groups of elderly (n=70) and young (n=70) sleep apneic patients, matched for disease severity (apnea/hypopnea index), body mass index and neck circumference. Elderly patients required lower CPAP levels compared to young [mean (sd): 6.9(1.9)cm H(2)O and 9.4(3.5)cm H(2)O, respectively; P<0.0001]. To investigate this finding, we studied the effects of CPAP and its components (inspiratory and expiratory positive airway pressure) on lung volume and upper airway resistance in two groups of elderly [n=9, age 71.7(3.3) years] and young [n=9, age 36.7(4.4)] patients with sleep apnea during wakefulness. CPAP produced a greater decrease in airway resistance (P=0.009) and a greater increase in lung volume (P=0.008) in the elderly compared to young patients. We conclude that both the greater lung inflation and the greater direct splinting of the upper airway contributed to the lower CPAP level required by the elderly. Ageing may be an important determinant of therapeutic CPAP levels in clinical practice, especially in older sleep apneic patients.