Hypophosphatemia, with or without phosphorus deficiency, is found commonly in patients with a variety of disease states. Severe hypophosphatemia is most often observed in chronic alcoholics, patients recovering from diabetic ketoacidosis and those being therapeutically re-fed after severe weight loss. Its consequences may be severe. In this review, an attempt is made to examine mechanisms by which hypophosphatemia and phosphorus depletion probably occur, to outline the resulting clinical disturbances and to suggest a plan of treatment.