Thyroid growth stimulating immunoglobulins microsomal antibodies and antibodies against thyroglobulin were determined in patients with simple goitre (n = 20) and controls (n = 6) living in an iodine deficient area. In addition, lymphocytic infiltration of thyroid tissue, the amount of the various lymphocyte subsets (Leu 4+, Leu 3a+, and Leu 2a+ T-cells as well as B1+ B cells) in the thyroid gland, as well as the expression of the histocompatibility antigen HLA-DR on thyrocytes and intrathyroidal T-lymphocytes were examined. Goitrous patients were subdivided into two groups according to their individual iodine supply estimated by iodine excretion values, and immunological parameters were compared between patients with low (group A, iodine excretion less than 70 micrograms/24 h) and with higher (group B, iodine excretion greater than 100 micrograms/24 h) iodine supply. Thyroid growth stimulating immunoglobulins and antithyroid antibodies were equally prevalent in the two patient groups, but were absent in controls. Lymphocytic infiltration of thyroid tissue was present to a comparable extent in patients of groups A and B, but to a distinctly lower degree in control persons. Intrathyroidal T-lymphocyte subsets did not differ between patients and controls. B-lymphocytes, germinal centres as well as DR+ thyrocytes were detected in goitrous patients of both groups, but never in control persons. Thus, immunological abnormalities frequently occur in patients with simple goitre and do not depend upon individual iodine supply.