The purpose of this study was to determine bedside criteria for discontinuation of mechanical ventilation. The resting minute ventilation (MV), maximal voluntary ventilation (MVV) and peak negative pressure on maximal inspiration (PNP) were studied in 100 consecutive patients receiving assisted ventilation. Seventy-six patients (Group 1) had MV less than 10 L/min and could double the MV with an MVV maneuver. All Group 1 patients could have mechanical ventilation discontinued. Seventeen patients (Group 2) could not double their resting MV with an MVV maneuver and required further ventilatory assistance. Seven patients (Group 3) could not double their resting MV with an MVV maneuver hut did not require further mechanical ventilation. Patients whose PNP was greater than 30 cm H2O less than atmospheric were always able to have mechanical ventilation discontinued. The ease with which these measurements can be performed at the bedside and their high degree of predictability make them useful in acute respiratory care.