Severe arterial hypoxia while breathing ambient air (Pao2 less than 60 mm Hg) was recorded in 38 (45 per cent) of a group of 84 patients with acute pancreatitis when arterial blood gas monitoring was performed during the first week of illness. This contrasted with an incidence of 19 per cent severe hypoxia in a heterogeneous group of 68 patients with acute abdominal pain admitted as surgical emergencies. Clinical evidence of respiratory embarrassment was rarely present in either the patients with acute pancreatitis or the other patients. Arterial hypoxia was related to the prognosis in both groups. It is recommended that all patients with acute pancreatitis have routine arterial blood gas monitoring performed during the first 5 days of illness. Furthermore, all older patients (over 60 years) and younger patients with moderate hypoxia (Pao2 less than 70 mm Hg) should be given supplementary humidified oxygen. An awareness of occult respiratory insufficiency in surgical emergency admissions, and especially in acute pancreatitis, is of importance.