Respiratory Function has been measured in 14 patients with acute pancreatitis, none of whom has a previous history of cardiac or respiratory disease. Their mean age was 40 years, mean PaO2 on admission, 11 kPa, and all had normal chest radiographs at that time. Arterial hypoxia was a common development and the mean PaO2 at the time of most severe hypoxia was 8.6 kPa with the lowest individual recording 4.9 kPa. In addition to daily arterial blood gas monitoring, dynamic ventilatory tests and measurements of lung volume, transfer factor, closing volume, right to left shunts and studies of gas exchange were performed. Gas exchange and shunt measurements indicated that the major abnormality responsible for arterial hypoxia was right to left shunting. Seven patients developed radiological abnormalities, five of whom has pleural effusions. Five patients showed a mild restriction in lung volume but there were no major consistent abnormalities of dynamic ventilatory tests, transfer factor, or closing volumes. Thus small airway closure, in dependent parts of the lung and less certainly loss of surfactant are unlikely to be major factors in the aetiology of arterial hypoxia in patients with acute pancreatitis. All measurements were repeated at least three months after the acute illness. The respiratory insufficiency which occurs in this disease is possibly another variant of the adult respiratory distress syndrome.