Four human fetuses were treated by transplantation of human fetal liver stem cells. Two of them had severe immunodeficiency disease and the two other ones had thalassemia major. Three of these in utero transplants were followed by engraftment. The three patients are now born: the first one is now very healthy thanks to the reconstitution of cell-mediated immunity associated with this transplant, and he lives normally at home; the two other ones, who have been more recently treated, have a significant improvement of their condition and they also live normally at home. This procedure, for the first time used in humans, has therefore demonstrated its feasibility and its efficacy: during early fetal development, foreign cells engraft readily and may result in cure or significant correction of a large variety of inherited diseases.