SUMMARY Two cases, one of 64 year old man and another of 65 old woman with severe hypophosphatemia, with phosphate serum level below 1 mgr/dl are presented.They were admitted to the Intensive Care Unit with severe respiratory failure requiring mechanical ventilation. They had a very difficult weaning, even when the reason of respiratory failure was promptly reversed. The correction of severe hypophosphatemia by intravenous administration of phosphate salts, resulted in a dramatic improvement of the contractile properties of the respiratory muscles followed by a successfull weaning from mechanical ventilation. Pneumon 1999, 12 (1): 39-45