Thirty-two patients were evaluated within 24 hours of admission for 36 episodes of acute respiratory failure (arterial oxygen pressure less than or equal to 50 mm Hg). Clinical data, spirometric determinations, blood gas analysis, and synchronization of chest (rib cage) and abdominal (diaphragmatic) breathing movements were studied. All patients were initially treated with controlled oxygen therapy. In 25 episodes the patients recovered without intubation (successes). In nine episodes the patients required intubation and assisted ventilation; two of these patients died. Two patients died without intubation. The 25 successful episodes were compared with the 11 requiring intubation or associated with death (failures). The breathing pattern proved to be the best single factor for predicting success or failure (77 percent correct prediction). The breathing pattern plus the arterial carbon dioxide tension on admission was the best two-factor guide (86 percent correct prediction). Patients with asynchronous breathing and severe hypercapnia are so unlikely to do well with a program of controlled oxygen therapy that preparations for intubation and assisted ventilation should be made on admission and such measures should be instituted at the first sign of deterioration.