Three human fetuses were treated by transplantation of human fetal liver stem cells. Two of them had severe immunodeficiency disease and the third had thalassaemia major. All three in-utero transplants were followed by engraftment. No adverse effect was seen. The three patients have now been born: the first is very healthy thanks to the reconstitution of cell-mediated immunity associated with this transplant, and he lives normally at home; the other two, who were treated more recently, have not yet shown a complete effect. The feasibility and efficacy of this procedure, used for the first time in humans, has therefore been demonstrated: during early fetal development, foreign cells engraft readily and may result in significant improvement of a large variety of inherited diseases.