Standard bedside criteria of respiratory mechanical capability and the mechanical work of spontaneous breathing were measured in 17 mechanically ventilated patients. Eleven patients were extubated within 24 hs of study and required only a brief period of mechanical ventilation (group 1). Group 2 consisted of six patients requiring more prolonged ventilator support. Group 1 patients met three of four bedside criteria; seven patients met all four. Five of six patients in group 2 also satisfied three of four standard criteria while ventilator-dependent, whereas only two patients satisfied all four when successfully weaning. As group 2 patients progressed from unsuccessful to successful weaning there was no consistent improvement in bedside criteria; however, measures of work did significantly improve. Hence, satisfaction of bedside mechanical weaning criteria is associated with weaning success in patients requiring brief mechanical ventilation. In patients requiring prolonged ventilation, work may be a better indicator of successful weaning.