Concordant Graves' disease after bone marrow transplantation: implications for pathogenesis. The current working hypothesis on the pathogenesis of autoimmune disease focuses on the interactions between susceptibility genes and environmental stimuli. In Graves' disease it is postulated that aberrant expression of HLA class II antigens on thyroid epithelial cells permits the presentation of specific thyroid antigen to activated lymphocytes. Evidence suggests that thyrocyte HLA-DR expression is secondary to the production of cytokines by presensitized T-lymphocytes. A 20-yr-old woman and her 18-yr-old brother presented with classical findings of Graves' disease with ophthalmopathy within a year of each other. Diagnosis was confirmed by demonstration of elevated serum levels of T4 and T3, strongly positive titers of TSH binding inhibitory immunoglobulins, and histological examination after subtotal thyroidectomy. Eight years previously, acute life-threatening aplastic anemia in the brother led to therapeutic transplantation of bone marrow from his sister. After the procedure, 100% of his peripheral leucocytes were genotype 46,XX. HLA typing performed before transplantation and 2 months after thyroidectomy in the female indicated complete identity with her brother's leukocytes for class I and class II antigens. Thyroid autoantibodies at this time were weakly positive. Although the concordance of thyroid disease in these patients could be due to chance, the patients were of different sexes, the family history was negative, and neither the probands nor the first degree relatives bore the HLA-DR3/B8 antigens. We propose that the male passively acquired a clone of programmed or activated lymphocytes from his sister and that his hyperthyroidism was not primarily dependent on exposure to specific thyroid-derived antigen. 