Twenty-six patients with chronic obstructive airflow disease (23 also complained of excessive daytime somnolence) were monitored during sleep, using noninvasive techniques, for evaluation of respiratory and sleep variables. Ten of the 26 had a second more extensive evaluation, including measurement of endoesphageal pressure; 5 underwent a study of nocturnal hemodynamics, with measurement of systemic and pulmonary pressures; 5 received O2 in air during sleep under controlled conditions. Eight of the studied patients underwent tracheostomy and were restudied postoperatively. Both sleep stage and type of abnormal respiratory event during sleep affected oxygenation levels. A mean of 92% of all abnormal respiratory events during sleep contained an obstructive component. Apneic events may affect systemic and pulmonary arterial pressures during sleep. Administration of O2 in air may, in some cases, be detrimental to the sleeping patient. Tracheostomy unexpectedly improved the daytime, awake blood gas readings of somee patients. The mechanism of daytime somnolence was not elucidated in all cases.