The source of TSH-receptor antibodies (TRAb) in Basedow's-Graves' disease is still under debate. Previous studies by other groups had found TRAb levels in the thyroid vein higher than or equal to that in the peripheral vein. The aim of the present work was to investigate the suspected presence of this TRAb gradient in 14 Graves' patients who underwent surgery. Interestingly, in 6 patients higher levels of TRAb were measured in the antecubital vein when compared to the thyroid vein, in another 6 there was no gradient and 2 exhibited higher TRAb levels in their thyroid veins. A clear gradient of thyroid hormones in favour of the thyroid vein was also present, while no gradient of TSH, anti-thyreoglobulin and anti-microsomal antibodies, total IgG, IgA and IgM levels and no differences of the CD4+/CD8+ cell ratios were found between the two sampling sites. We conclude that cells other than lymphocytes residing in the thyroid gland must also be involved in TRAb production.