Henry VIII's first two wives experienced multiple pregnancies culminating in late-term miscarriage, stillbirth, or neonatal mortality. After his fortieth birthday, the king's mental and physical health underwent rapid deterioration. In this article, we argue that both his reproductive troubles and his midlife pathologies can be explained if Henry VIII were positive for the Kell blood group. A Kell negative woman who has multiple pregnancies with a Kell positive male will suffer repeated miscarriages and death of Kell positive foetuses and term infants that occur subsequent to the first Kell positive pregnancy. This pattern is consistent with the pregnancies of Katherine of Aragon and Anne Boleyn. Additionally, Henry VIII may have suffered from McLeod syndrome, a genetic disorder of the Kell blood group system, which is a condition that causes physical and mental impairment consistent with his ailments.