A 4-h infusion of 310 to 465 mg (10 to 15 mMol) phosphorus given to 28 of 31 consecutive seriously ill hypophosphatemic patients increased the serum phosphorus level above 1.2 mg/dl in all but one patient. There was no significant change in the mean serum calcium, potassium or blood pressure, no patient deteriorated, and six patients were stronger and more alert after the infusion. In seriously ill patients we recommend a 4-h infusion of 15 mg/kg (0.5 mMol/kg) phosphorus if the serum phosphorus is less than 0.5 mg/dl, or a 7.7-mg/kg (0.25 mMol/kg) infusion if the serum phosphorus is between 0.5 and 1.0 mg/dl.