An infant with idiopathic hypoparathyroidism was found to have no evidence of thymus by radiologic examinations, including pneumomediastinum. Immunoglobulin concentrations were normal but there was clear evidence of inadequate cellular immunity including gradually developing lymphopenia. The infant suffered recurrent diarrhea and pneumonia, chronic rhinorrhea and moniliasis. At age 7 months fetal thymic tissue was implanted in the rectus abdominis muscle. There was immediate increase in lymphocyte counts which have remained normal on most determinations. Other aspects of immunologic function were restored and remain intact. The child, now 6 years of age, although mentally retarded, has no unusual infections. Immunologic reconstitution was thus achieved by transplant of fetal thymus.