Spirometry, blood gases, steady state diffusing lung capacity for carbon monoxyde, ductance for carbon monoxyde were determined in 27 obeses. In 13 of them we also measured closing volume. In 80 per cent of these patients we observed a decrease in expiratory reserve volume certainly due to a shift on the right of the chest wall volume-pressure curve. In 50% of the obeses we found an arterial hypoxia, usually observed when the expiratory level was lower than closing volume. Finally in 20% of the subjects we observed a decrease in diffusing lung capacity and/or ductances probably due to an important fall in and expiratory level at the closing volume level with impairment of the distribution of ventilation.