Hypophosphatemia is common in hospitalized patients and occurs under a variety of circumstances other than parathyroid hormone excess. Charts of 100 inpatients with hypophosphatemia were reviewed and the patients divided into five groups on the basis of serum phosphate level: 18, 2.1 to 2.4 mg/dL; 49, 1.6 to 2.0 mg/dL; 20, 1.1 to 1.5 mg/dL; 12, 0.6 to 1.0 mg/dL; 1, 0.1 to 0.5 mg/dL. The effect of glucose ingestion on serum phosphate level was shown in one normal patient. Whenever carbohydrate was administered intravenously (45 cases), this was considered the primary cause of the hypophosphatemia. Other causes were as follows: diuretics, hyperalimentation, alcoholism, respiratory alkalosis, dialysis, insulin, corticosteroids, diabetic ketoacidosis, vomiting, phosphate-binding antacid, Gram-negative sepsis, primary hyperparathyroidism, saline, epinephrine, gastrointestinal malabsorption, and unknown. Hypophosphatemia in hospitalized patients may have multiple causes.