Definition of the problem The allogeneic transplantation of haematopoietic stem cells from the body of small children, who are unable to consent to the donation, to a sick sibling raises difficult ethical questions. How can a non-therapeutic medical intervention for the benefit of others that is not free of risks be ethically justified? This paper critically examines arguments of duty, which claim that the donating child has an obligation to donate. The idea of a duty can clearly be understood from the perspective of parents, who find themselves in a situation of conflicting duties of care (to injure one of their children in order to save the life of the other). The parentsdo have a duty to fight for the life of their sick child.Arguments An analysis of the notion of duty shows that in order to be bound by a duty one must be considered competent to decide. Therefore, the paper argues that children who have been denied such competency cannot be said tobe bound by a duty to donate blood stem cells. Then we examine three ways toascribe a duty to the child: (i) the parents transfertheir duty to their child; (ii) the best interests of the donor child imply a duty to donate; (iii) on the basis of intimate family relationships siblings share a set of duties, including the obligation to save a life by donating blood stem cells. All three strategies are not without criticism.Conclusion It is not possible to ascribe a duty to donate body tissue to a sibling to an incompetent minor. Other ethical approaches to blood stem cell transplantation from minors to family members must be developed and evaluated.