To compare the effectiveness of two routinely used methods of preoxygenation in protecting against hypoxia in the elderly, the arterial O2 saturation was measured using an oximeter. Twenty-four elderly patients (greater than or equal to 65 yr) presenting for elective orthopedic surgery were randomly allocated to receive either 3-min or four-maximal-breaths of 100% O2 via a Bain circuit. After preoxygenation, anesthesia was induced, tracheal intubation performed with patients kept apneic, and the endotracheal tube left open to air. The arterial O2 saturation was measured before preoxygenation and continually recorded during desaturation. Although attaining similar arterial O2 saturation values after preoxygenation, patients in the four-maximal-breath group had significantly shorter times (P less than 0.0001) to all levels of desaturation. We suggest that preoxygenation with 3-min breathing of 100% O2 offers more protection against hypoxia due to prolonged apnea after induction of anesthesia in the elderly than does four maximal breaths of 100% O2.