1. Measurements of lung volumes, airway resistance, maximal expiratory flow rates and closing volume were made in twelve patients with chronic renal insufficiency and fluid overload, before and after haemodialysis.
2. Before dialysis, these measurements were within normal limits, except for peak expiratory flow rate, which was significantly lower, and closing volume, which was significantly higher than predicted values.
3. After dialysis body weight decreased significantly and the removal of excess of fluid was accompanied by a significant decrease of closing volume.
4. Measurements of closing volume and the other pulmonary tests were made in six healthy subjects, the same timing being observed as in the patients. Mean as well as individual values of closing volume did not alter, thus excluding the possibility that the changes observed in the patients were due to a circadian rhythm.
5. It is suggested that the higher than predicted closing volume was due to the accumulation of fluid in the dependent areas of the lung, promoting a premature airway closure. Removal of excess of fluid allowed airways to close at a lower lung volume.