Phosphorus (P) is a basic element that is found throughout the body. Most of the body’s phosphorus is bound to calcium in the bones, but about 15% exist in the blood and other soft tissues and body fluids. Phosphorus is very important in metabolism. More than 80% of the body’s phosphorus exist as calcium phosphate in the skeleton; the remainder is a component of phospholipids,nucleic acids, and ATP. Nearly all serum phosphorus is present as inorganic phosphate, levels of which are controlled by parathyroid hormone, vitamin D,and calcitonin. Hyperphosphatemia has been observed in bone metastasis,hypocalcemia, hypoparathyroidism, liver disease, sarcoidosis, Addison’s disease, hypervitaminosis D, magnesium deficiency, myelogenous leukemia,and renal failure. Hypophosphatemia may occur in rickets, osteomalacia,diabetic ketoacidosis, hyperparathyroidism, hyperinsulinism, hypercalcemia,vitamin D deficiency, Gram-negative sepsis, and alcoholism, and in patients taking diuretics, corticosteroids, or aluminum hydroxide antacids.