The sooner oxygen is employed for relief of myocardial hypoxia in acute coronary thrombosis, the greater is its life-saving potential, a view supported by recent studies. In two patients, convalescent from coronary thrombosis, the arterial pO2 was more than doubled upon inhalation of 40% oxygen with a pocket dispenser. Electrocardiographic abnormalities induced by the Master 2-step exercise test while breathing air were inhibited, partly or totally, in 11 of 12 patients when the exercise was performed breathing oxygen. Recently-developed portable equipment for effective oxygen therapy includes refillable 75- and 150-liter cylinders with 2-stage regulators, as well as smaller devices with disposable, miniature oxygen cartridges. This apparatus, which can be carried by doctor and patient, has now made feasible an earlier and more widespread therapeutic use of oxygen.