It is generally agreed that one assisted ventilation session has direct effects in chronic respiratory insufficiencies (C.R.I.) of chronic obstructive pulmonary diseases: hyperventilation where PaCO2 decreases and PaO2 increases, diminution of ventilatory work, reduction of cardiac output. Less is known, however, about the action on V and Q distribution and mechanics of breathing. The return to control values occurs quite rapidly after the end of the session. Long-term effects are much more questionable. Many publications in English have concluded that ona long-term basis, patients do not benefit from intermittent positive pressure. But, the daily assisted ventilation sessions usually lasted only a short time (1 h-1 h 30 in several sessions). According to a number of publications in French, assistance prolonged for several hours seems effective (decrease in the number of acute failures, annual hospitalization duration, more prolonged survival). However, a tracheostomy which enables assistance at night is often required. Results seem encouraging in the treatment of severe restrictive insufficiencies and particularly in the case of cyphoscoliotic patients. On the other hand, it is more difficult to draw a conclusion concerning chronic obstructive patients and further comparative studies are necessary in this direction.