Although severe hypophosphatemia has been recognized in refeeding syndromes, it is not a commonly reported complication of enteral nutrition. The present study was designed to identify cases of severe hypophosphatemia (less than 0.32 mmol/L [less than 1.0 mg/dL]) related to the administration of carbohydrates via the enteral route. Serum phosphorus levels were evaluated at the time of admission of 25 patients to two midwestern teaching hospitals and during their postoperative enteral support in the surgical intensive care unit. The initial serum phosphorus levels ranged from 0.77 to 1.55 mmol/L (2.4 to 4.8 mg/dL), serum calcium levels ranged from 1.80 to 2.44 mmol/L (7.2 to 9.8 mg/dL). From two to five days following the initiation of isotonic enteral feedings, the serum phosphorus level decreased to 0.16 to 0.39 mmol/L (0.5 to 1.2 mg/dL). Serum phosphorus levels were corrected within two to ten days with oral supplementation only. Patients with high metabolic demand may have a higher daily requirement for phosphorus than that available in routine isotonic enteral formulas.