10 patients thymectomized about 20 years earlier were tested for delayed skin reactivity against mumps, streptokinase-streptodornase and PPD antigens. The T and B lymphocytes in peripheral blood were determined. 6 patients were surgically thymectomized and 2 of the received postoperative irradiation. 4 patients were treated with only high dose irradiation to the mediastinal compartment. None of the patients responded to all skin antigens, 1 was positive to two antigens and 3 to one antigen. The remaining patients were non-responders. The number of peripheral blood lymphocytes was slightly decreased in the patients. A significant decrease in the number of T-lymphocytes (p less than 0.005) was found in the whole patient group, and the depression was more pronounced in the surgically thymectomized patients (p less than 0.001). The number of B-lymphocytes was unaltered. It is concluded that the abnormalities in this patient group reflect a T-lymphocyte deficiency. The immunologic status of the patients before treatment is not known. However, in 3 patients with recently discovered untreated thymomas no abnormalities were found. Thus, adult thymectomy in man seem to lead to deficiency of cell mediated immunity, indicating an influence of the thymus in immunological functions during adult life.