The introduction of the concept of intermittent mandatory ventilation (IMV) has meant that for the first time weaning is controlled. The abrupt cessation of ventilatory support, even for short periods, as was obligatory before IMV, is no longer necessary. The frequency of positive pressure inflations can be gradually and progressively decreased as indicated, leading to spontaneous ventilation in a planned, controlled program. The ease with which other artificial ventilation technics may be incorporated into this system adds to its usefulness. IMV should be considered whenever ventilatory support is to be discontinued.