Airway closure (closing capacity, CC), FRC, total efficiency of ventilation (lung clearance index, LCI) and distribution of inspired gas (nitrogen washout declay percentage, NWOD) were determined by nitrogen washout techniques and arterial PO2and PCO2 measured by standard electrodes in 10 extremely obese subjects, prior to an during anaesthesia and artifical ventilation. CC was normal, but because of small FRC, airway closure occurred within a tidal breath in 9 out of 10 subjects during spontaneous breathing, when awake. PO2 was reduced, the hypoxaemia correlating to the magnitude of airway closure. LCI was normal, but NWOD was borderline. During anaesthesia, CC was unaltered by FRC was further reduced, so that in nine subjects sirway closure occurred above FRC and tidal volume together. A marked increase in relative hypoxaemia was recorded. LCI and NWOD rose, indicating less efficient and less even ventilation. It is concluded that airway closure reasonably explains the marked hypoxaemia in obese subjects during anaesthesia, and that it may also be the reason for the uneven distribution of inspired gas.