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 refed after severe weight loss. Its consequences may be severe and contribute to mortality. The following represents a selective review of available literature on this subject published over the past 50 years and a limited number of observations on patients and experimental animals. An attempt is made to examine mechanisms by which hypophosphatemia and phosphorus depletion probably occur, outline the resulting clinical disturbances, and suggest a plan of treatment. Finally, this review will hopefully point out areas of this interesting deficiency state that desperately need further investigation.