The thymus is a gland situated in the lower neck and upper part of the chest behind the upper breastbone (sternum) and in front of the windpipe. It is a part of the lymphatic system. This gland weighs at birth about 15 grams, in puberty about 35 grams, and is very active in the early phase of life. Lymphocytes from the bone marrow divide in the thymus and become T-lym-phocytes. During adult life the thymus gradually diminishes in size and by about the age of 60 years it is largely replaced by fat. For this reason the thymus is often regarded as a temporary gland. However, it plays a crucial role during childhood. If the thymus fails to develop, the child has a deficiency of T-lymphocytes and usually dies from irresistible infections. Sometimes transplantation of thymic or bone marrowcan treat this condition. Because of absence of an original thymus, there is no incompata-bility and the transplant is not rejected.