The characteristics of the normal adult thymus in both sexes were determined in 50 cases of accidental death by a simplified quantitative histologic technic. A table of normal values derived from these findings was used for making comparisons with thymus from autopsies of 50 additional patients suffering terminal illness. It was observed that thymic changes considered to be part of physiologic involution occurred earlier and were more advanced at a given age in males than in females. These changes included disappearance of septae, diminished demarcation between cortex and medulla, and a decreased number of Hassall's corpuscles with an increase in their size. Changes ascribed to disease included accelerated involution of the thymus accompanied by loss of septae, smaller lobules, increased adipose tissue and fusiform cells, a reduced number of lymphocytes and Hassall's corpuscles and a relative increase in the number of cystic corpuscles. No lymphoid follicles were observed. Changes were not identical in all disease conditions. In neoplasia, the limit between the cortex and medulla was preserved; in immune diseases, the epithelial nests of the medulla were increased.