Arterial blood gases (pH, pO2, p CO2) were studied in 100 patients with documented pulmonary embolism (Group A), confirmed by pulmonary angiography (n = 51) or scintigraphy ( n = 49). The pO2 ranged from 32 to 97 mm Hg (average 60,5 +/- 13 mm Hg). Hypoxaemia was found in 97 cases and would therefore seem to be a reliable sign of pulmonary embolism. In the three cases in which it was absent, the embolism was small. Hypoxaemia was associated with hypocapnia and alkalosis in 91 cases. However, hypoxaemia was not a specific finding; it was also present in 49 patients with suspected pulmonary embolism (Group B) in whom the diagnosis was excluded by pulmonary angiography or scintigraphy. A previous history of cardiovascular disease was found in 37 patients (76%) in this group: of the 12 remaining patients, 6 were heavy smokers and 4 were significantly obese. No correlation was found between the degree of hypoxaemia and the extent of amputation of the vascular bed on pulmonary angiography or scintigraphy. Nevertheless, a pO2 of under 50 mm Hg was always associated with a severe embolism with amputation of over 40% of the pulmonary vascular bed. A significant correlation was found between the severity of hypoxaemia and the degree of cyanosis (p less than 0,05) and ECG changes (p less than 0,01). The average pO2 was 59 +/- 12 mm Hg in patients with cardiovascular disease ( n = 21) and 55 +/- 11 mm Hg with known pulmonary disease ( n = 6). A higher average pO2 was found when these conditions were absent (61,5 +/- 13 mmHg). The difference was not statistically significant unless previous cardiac and pulmonary disease were associated (pO2 = 51 +/- 14 mm Hg, p less than 0,05).