The function of high endothelial venules (HEVs), present in the T-cell area of lymphoid tissue is to attract lymphocytes to secondary lymphoid organs ("homing"). In Graves' disease, sporadic goitre and lymphocytic thyroiditis HEVs develop in the thyroid. To study the "homing" of peripheral blood lymphocytes (PBL) of healthy individuals and thyroid patients to the thyroid area we studied the adherence of PBL of such individuals to HEVs present in Hashimoto's goitres and to HEVs in thyroid draining lymph nodes. A modification of the in vitro "homing assay" described by Stamper and Woodruff (J Exp Med 144: 823) was used. The number of PBL of patients with Graves' disease which adhered to thyroidal and thyroid-draining lymphnode HEVs was significantly (p less than or equal to 0.001, Wilcoxon test) less than that of healthy control PBL's; in the case of thyroid HEVs 12 (mean, sd 8, n = 18) patient lymphocytes adhered to 35 HEVs vs 19 (mean, sd 7, n = 16) healthy control lymphocytes; in the case of thyroid lymphnode HEVs 20 (mean, sd 12, n = 15) patient lymphocytes adhered vs 35 (mean, sd 9, n = 9) healthy control lymphocytes. PBL of a few sporadic goitre (n = 5) and atrophic lymphocytic thyroiditis (n = 2) patients also showed a diminished adherence to thyroidal HEVs. We also studied the homing capability of lymphocyte suspensions isolated from the thyroid glands of three Graves' disease patients; these infiltrated cells showed a normal adherence pattern to thyroidal HEVs. We favour the idea that the data should be explained by a redistribution of lymphocytes possessing "thyroid-specific-homing-receptors" from the circulation to the thyroid area in patients with thyroid autoimmune disease.