Hashimoto's thyroiditis is known to occur in conjunction with other autoimmune disorders including rheumatoid arthritis. We describe herein a patient with long before-onset seronegative rheumatoid arthritis who developed Hashimoto's thyroiditis and hyperthyroidism without serologic evidence of thyroglobulin, microsomal and/or anti-TSH receptor antibodies. The occurrence of these autoimmune diseases in individual patients suggests an imbalance in immune function which effects more than one organ system. The predisposition to this spectrum of autoimmune diseases may be genetically determined, with specific HLA haplotypes associated with a variety of autoimmune diseases. The case of this patient provides the demonstration that intrathyroidal lymphocytes in autoimmune thyroid disorders and T-lymphocytes in the synovium are responsible for mediating the glandular destruction in Hashimoto disease and intraarticular lesions in rheumatoid arthritis, since the disorders can exist without evidence of a systemic immune response.