The significance of intrathyroidal lymphocytic infiltration is not known. However, several indirect lines of evidence suggest that interstitial or intraepithelial lymphocytes are the effector or thyroid autoantibody-producing lymphocytes in Graves' disease. This has not been investigated in vivo. Changes of nuclear volume of endocrine cells have previously been shown to be a reliable parameter of functional stimulation of endocrine glands. Therefore we investigated this parameter near and off lymphocytic aggregates, loosely distributed plasma cells and memory T cells in paraffin sections of Graves' disease thyroid glands. In 21 Graves' disease thyroid glands we found significant increases of thyroid epithelial cell nuclear volume near plasma cells (198.4 micron3) as well as near lymphocytic aggregates (219.1 micron3) compared with thyroid epithelial cell nuclear volume one microscopic field away (160.1 and 137.7 micron3 respectively). Similar nuclear volume differences were observed after propanolol and thiourelene antithyroid drug treatment. These nuclear volume differences could not be observed in 10 control thyroid glands and around CD45R0-positive memory T cells in Graves' disease thyroid glands. These direct in vivo investigations of regional functional stimulation of thyroid epithelial cells in Graves' disease show local stimulation near lymphocytic aggregates and diffusely distributed plasma cells. Therefore our in vivo data do not permit to identify stimulatory lymphocytes only interstitially or intraepithelially as previously suggested.