So far the cases considered have concerned making decisions for individual children. When parents make decisions for their incompetent children, I have suggested that it may be possible for reasonable parents to take into account a range of factors, which potentially could include the needs of other family members (see, also, Ross, 1998). The extent to which this should be permissible is controversial, but the issue comes into sharp focus where treatment for one child involves a medical procedure being performed upon another. This is most likely to occur where bone marrow or organ donations between siblings are proposed. The need to consider intrafamilial donations has become more pressing with the development of methods of assisted reproductive technology and pre-implantation genetic diagnosis. These make it possible for a couple to conceive a child that could then act as a tissue-matched donor for a sick brother or sister.1 In the absence of effective alternatives, it can be expected that sibling donations, particularly involving very young infants, will be sought in increasing numbers. With older and competent children, the question of whether they should be able to make the decision to donate will also need to be asked. A more rare situation where medical interventions might be undertaken to benefit another child is that of conjoined twins. The most dramatic example of this in the UK has been the case of Jodie and Mary, where surgical separation was necessary to save the life of one, but would inevitably kill the other.2 What these situations have in common is that medical procedures are being proposed to meet the needs of one child, but that have no medical benefit and in some cases considerable risk, even extending to the certainty of death, to another. If the proper test for the courts is the best interests of the child, how can this be applied when two children are involved and their best interests appear to be incompatible? These issues will be considered here.