A fatal progressive wasting disease was produced in male hamsters by thymectomy. The disease was characterized by generalized atrophy of lymphoid tissue, pancytopenia and decreased gamma globulins. Two unique features of this wasting disease were its occurrence in a significant number of animals thymectomized at 4 weeks of age and its development in males only. The thymectomized female hamster could be made to waste if treated with testosterone proprionate or bilateral oophorectomy. The mechanisms involved in the post-thymectomy wasting disease are discussed, as well as the relationship of runt disease (graft vs. host disease) to thymic wasting. The relationship of the thymus to lymphoid tissue, immune globulins, and certain diseases in man are pointed out. The possibility that the wasted thymectomized hamster may be an experimental model for the human disease known as "alymphocytosis" or "lymphocytophthisis" is discussed.