Based on the prevailing interpretation of recent data implicating donor homozygosity for an HLA haplotype shared with a heterozygous recipient, it is our conclusion that the risk of TA-GVHD for a sibling of the blood donor is about half that of a parent or a child, and the risk for a grandparent, a grandchild, or a blood-related aunt, uncle, niece, or nephew is about the same as the risk for the donor's sibling. Consequently, the current recommendation and the practice of irradiating cellular blood products for transfusion to first-degree relatives but not for transfusion to more distant relatives is insufficient. We believe that the recommendation for prophylactic irradiation of cellular blood products, based on the current understanding of its apparent safety, should be extended at least to include transfusion to the donor's grandparents, grandchildren, and blood-related aunts, uncles, nieces, and nephews.