Liver transplantation has been reported in a few cases of maple syrup urine disease (MSUD), but is controversial. Many patients with approved indications for liver transplantation die before grafts are available. A 25-yr-old man with MSUD underwent liver transplantation, and his liver was used as a domino graft for a 53-yr-old man with hepatocellular carcinoma who had low priority on the liver transplant waiting list and was unlikely to survive until routine organ procurement. Both transplants were performed as "piggy back" procedures, reconstructing the domino graft with caval segments from the cadaveric donor. Neither required veno-venous bypass. Whole body leucine oxidation was estimated by 13CO2 in breath after oral boluses of L-[1-13C]-leucine, before and after transplantation in both patients and a control subject. The surgical outcome was successful. The patient with MSUD had marked decreases in plasma branched-chain amino acids (BCAAs) and alloisoleucine (from 255 +/- 66 to 16 +/- 7 micromol/L), despite advancement of dietary protein from 6 to >40 gm/day. The domino recipient maintained near-normal levels of plasma amino acids with no detectable alloisoleucine on unrestricted diet. Leucine oxidation increased in the patient with MSUD (from 2.2 to 5.6% recovered in 4 hours) and decreased in the recipient (from 9.7 to 6.2%). Neither patient demonstrated any apparent symptoms of MSUD over more than 7 months. In conclusion, liver transplantation substantially corrects whole body BCAA metabolism in MSUD and greatly attenuates the disease. Livers from patients with MSUD may be considered as domino grafts for patients who might otherwise not survive until transplantation.