BACKGROUND: During orthotopic liver transplantation, magnesium monitoring has been done frequently because of its important role in the cardiovascular system. Generally hypomagnesemia may occur frequently during an operation, but some investigators reported serum magnesium levels returned to normal after reperfusion which is different from our result. Therefore, this study was done to confirm the changes of serum magnesium in the postanhepatic stage and also to confirm the need for prophylactic magnesium administration. METHODS: Thirteen patients plasma magnesium concentrations were measured 8 times during the operation on each patient. We also checked the total transfusion amount and required CaCl2 amount in every patient. The significance of all data were evaluated with a paired t-test and correlation method. RESULTS: Serum magnesium levels were significantly decreased after the beginning of the postanhepatic stage during an operation (P < 0.05). Correlation between transfused blood amounts and CaCl2 requirements showed significant correlation (r: 0.709), but no correlation between magnesium concentrations and transfused blood amounts was found. CONCLUSIONS: We concluded that serum magnesium concentrations were significantly decreased especially in the postanhepatic stage and an appropriate amount of a magnesium supply may be needed.