SummaryThirteen cases are reported in which decreased respiration in the middle lobe of the lungs was produced by bronchial obliteration of inflammatory origin.In two cases the disease was characterized by total and well localized opacity of the middle lobe.In eleven cases X ray examination showed only those pictures generally described as«fibrosis of the right basis»or as«accentuation of bronchovascular shadows».Bronchography is the best method to study those pictures. It shows a defect in the filling of the middle lobe bronchus or of one of its branches.It has been shown that the disease has its origin in a functional disturbance which is, in the lower lobes of the lungs corrected by the possibility of collateral supply between pulmonary lobules at that level.It was possible, by bronchoscopy, to determine the exact cause of hypoventilation in 3 of 8 cases.In one case localized bronchitis was found, in two others obstruction was produced by adhesive bronchial secretions in its lumen.May be in the five other cases the same factors had to be incriminated at a point that could not be approached by bronchoscopy.No pathological study was possible.The prognosis seems to be favourable, although hypoventilation does not seem to disappear easily when present. Future must be reserved as possibility of later bronchiectases exists.