Hypophosphatemia was found in 12 (43%) patients within 24-36 h of mechanical ventilation in a group of 28 patients in an intensive care unit. The hypophosphatemic patients had a significant lowering of renal phosphorus threshold (P = 0.005) and inappropriate phosphaturia. These abnormalities resolved once ventilation was discontinued. Serum parathyroid hormone (PTH) levels were unaltered in this group of patients suggesting that some factor other than PTH was responsible for hypophosphatemia. This study has demonstrated that abnormalities of renal phosphate handling can cause hypophosphatemia in ventilated patients.