Phosphorus is a major component of proteins, phospholipids, and nucleotides. The increased uptake of phosphorus by cells during erythropoiesis can result in severe hypophosphatemia. A case of severe hypophosphatemia due to accelerated erythropoiesis in response to Cefotetan-induced hemolytic anemia is described. The hypophosphatemia seen during hemolysis may be the result, rather than the cause, of the hemolysis.