The thymus is the major site of T cell production and a key organ of the immune system. Its natural involution during the course of life has cast doubts as to its importance for the integrity of our immunity in adulthood. We provide here an overview of the recent works focusing on the immunological evaluation of subjects thymectomized during early childhood due to cardiac surgery of congenital heart defects. These studies represent new advances in our appreciation of the role of the thymus in humans and more generally in our understanding of the development of immunosenescence.