• An 8-year-old girl with combined immunodeficiency secondary to adenosine deaminase deficiency developed thyroid failure of probable autoimmune origin manifested by linear growth deceleration, marked bone-age delay, and myxedema. To our knowledge, this association has not been previously reported. Immunologic abnormalities included absolute T-cell lymphopenia and markedly reduced in vitro lymphocyte responses to phytohemagglutinin and to alloantigen in the mixed lymphocyte reaction. The diagnosis of autoimmune thyroid disease was suggested by the presence of antithyroglobulin antibodies in the serum and by decreased, patchy uptake of iodine 123 on a thyroid scan. Autoimmune thyroid disease may have developed because the immunodeficient state, with its greater deficiency of suppressor/cytotoxic T cells, allowed expression of a clone of helper T lymphocytes specific for thyroidal antigens. Thus, autoimmune disease may be more common in immunodeficient states and appropriate surveillance should be instituted.