Airway closure (CV), functional residual capacity (FRC) and the distribution of inspired gas (nitrogen washout delay percentage, NWOD %) and arterial oxygen tension (PaO2) was measured by standard electrodes in eight extremely obese patients before and after weight loss (mean weights 142 and 94 kg, respectively) following intestinal shunt operation. Prior to weight loss, airway closure occurred within a tidal breath in six out of eight patients. This was associated with a low FRC, and NWOD % was higher than in the non-obese. PaO2 was lower than normal. CV was unchanged after weight loss, but due to an increase in FRC, airway closure did not occur within a tidal volume and NWOD % was normal. PaO2 was also normal. It is concluded that weight reduction in extremely obese patients improves arterial oxygenation because of an increase in FRC, whereby CV no longer occurs within a tidal volume. Intrapulmonary gas mixing becomes more even.