Life-threatening hypophosphatemia (phosphorus < 1.0 mg/dL) has been reported only once after liver resection for tumor and was associated with a significant increase in postoperative complications. Hypophosphatemia is associated with reversible cardiac dysfunction, hypoventilation, and impaired immunity. The purpose of this study was to determine the incidence of hypophosphatemia after elective right hepatic lobectomy for live donor adult liver transplantation (LDALT), investigate the associated complication rate and surgical outcome of live liver donors, and determine the efficacy of prospective treatment with phosphate repletion as part of total parenteral nutrition (TPN). Evaluation of 30 donors who provided 30 right-lobe grafts between December 1998 and January 2000 was performed. Of the initial 18 live liver donors (group 1), 10 donors were treated with TPN that contained slightly more (35 +/- 8 mmol/d) than the recommended daily allowance (RDA) of phosphorus (30 mmol/d) starting on postoperative day 1. The last 12 donors (group 2) were prospectively studied and administered similar TPN with 2 times the RDA for phosphorus (60 mmol/d). All donors in group 1 developed hypophosphatemia that was either life threatening (phosphorus < 1.0 mg/dL) in 70% or severely depleted (phosphorus, 1.5 to 1.1 mg/dL) in 30%. With more aggressive phosphate repletion (group 2), only 8% developed life-threatening (phosphorus < 1.0 mg/dL) hypophosphatemia and 30% developed severe (phosphorus, 1.1 to 1.5 mg/dL) hypophosphatemia. Results suggest that hypophosphatemia is a universal event after LDALT and may have contributed to the observed complications in this study. Repletion of phosphorus at twice the RDA abrogates the incidence of hypophosphatemia and may reduce donor morbidity. Institutions performing LDALT should carefully monitor live liver donors for hypophosphatemia and correct abnormal phosphate levels. Additional studies are needed to determine whether more aggressive parenteral repletion can prevent postoperative hypophosphatemia and thus improve outcomes.