Thirty-four members of a single family were studied and 9 of them were found to be suffering from hyperthyroidism associated with diffuse goitre. Exophthalmos was absent and transmission seemed to be independent of HLA type. Four of the 9 were studied prior to treatment but in all cases serum immunoglobulin levels were normal, antithyroglobulin and antimicrosomal antibodies absent, thyroid stimulating antibodies negative and the lymphocyte transformation responses to mitogens not different from those of controls. The results of testing the euthyroid family members were similarly negative, except in the case of a woman with type I diabetes mellitus who showed a low titre of antimicrosomal antibody. Seven of the patients underwent subtotal thyroidectomy. Lymphocytic infiltration of the excised portion was rarely present. Four of the glands were subjected to immunofluorescent and electron microscopy but neither immunosecreting cells nor immune complex deposits were found. These results point to the existence of a non-autoimmune form of goitrous hyperthyroidism, different from Graves' disease.