One hundred and thirty-six thymuses completely removed at autopsy from persons suffering a sudden death were examined by stereological and morphometrical methods. Adding biopsy material from immunologically healthy cardiac patients we obtained relative volumes from 204 persons ranging in age from 1 month to 107 years. The size of the human thymus remains unchanged during ageing under normal physiological conditions (median: 19.5 cm3). Individual maximum size (range: 5-70 cm3) is reached in the first year of life. Early histological changes are in the enlargement of the perivascular space, the Hassall's bodies, and the connective tissue. This begins in the first year of life, reaches a maximum from 10 to 25 years, then declines again. Adipose tissue replaces the lymphocytic perivascular space and the connective tissue only. This occurs extensively after the age of 15 years. When defined by the silver impregnation technique, the volumes of the thymic epithelium (cortex and medulla), show a continuous involution from the first year to the end of life. The curve can be approximated to simple negative logarithmic functions. The velocity and nature of involution of the thymic epithelium do not change under the influence of the changing hormonal balance due to puberty. Since important thymic functions (T lymphopoiesis and T-cell differentiation) are located in the epithelial space, the age-related involution of the human thymus is not related to puberty.