We report a case in which a healthy 18-year-old girl acted as a liver donor for her mother, who had a terminal hepatic illness with a one-year survival probability of 2% without liver transplantation. The daughter,who was the only relative to have a blood type compatible with her mother,offered to donate after the surgeons explained the situation. Our ethical committee required a psychiatrist to evaluate her psychiatric status and competence, and address ethical concerns. We assessed elements of informed consent : disclosure of information, competence, and voluntariness, as well as psychiatric problems and the psychosocial status. The disclosure of information was adequate. The girl's competence was of a sufficient level for her to appreciate that the procedure could have negative results on both her and her mother. She was able to logically assess the consequences of having the operation and those of declining it. It was very difficult to evaluate whether she was undergoing the procedure completely voluntarily, because she was the only one with a blood type compatible with her mother, who was near death, and because the girl was underage. Psychological issues related to her age were that she depended on her family economically and psychologically, and that the opinions of her family influenced her. In this case, we concluded that she had consented voluntarily, because there was no coercion and no unjust pressure to donate,and because she understood that consent could be withdrawn at any time. Although she was underage and the psychological assessment revealed that she experienced some conflict in this difficult situation, her competence was considered intact. Medical evaluation and laboratory testing were satisfactory, and the ethical committee determined that she was suitable to donate. The operation was performed and was successful. The Japanese Transplantation Society has since established ethical guidelines regarding potential living donors who are underage; one precondition for donation is that a psychiatrist must evaluate whether or not the competence of the donor is equal to that of an adult. As the literature contains no reports of the evaluation of potential donors who are underage,we need to consider the format and contents of such psychiatric evaluations and ethical concerns.