We studied the blocking type TSH receptor antibodies in 28 patients with primary myxedema and 21 patients with goitrous Hashimoto's thyroiditis by measuring the ability of their IgGs to inhibit TSH binding to its receptor, and to inhibit TSH-stimulated cAMP increase and [3H]thymidine incorporation in a rat thyroid cell line, FRTL-5. The incidences of TSH binding inhibitor immunoglobulin, thyroid stimulation inhibiting immunoglobulin and thyroid growth inhibiting immunoglobulin in patients with primary myxedema were 54.6, 75 and 65.2%, respectively, against 14.3, 0 and 17.7%, respectively, in goitrous Hashimoto's thyroiditis. The antibodies inhibited dose-dependently not only TSH stimulated but also Graves' IgG-stimulated cAMP increase and [3H]thymidine incorporation. The TSH binding inhibitor immunoglobulin activities in patients with primary myxedema were significantly correlated with both the thyroid stimulation inhibiting immunoglobulin (r = 0.665; P less than 0.01) and the thyroid growth inhibiting immunoglobulin (r = 0.618; P less than 0.01) activity. Thirteen patients whose TSH binding inhibitor immunoglobulin activities were more than 50% had both strong thyroid stimulation inhibiting immunoglobulin (75.1-100%) and thyroid growth inhibiting immunoglobulin (57.4-100%) activities. These data suggest that the vast majority of patients with primary myxedema have potent blocking type TSH receptor antibodies. These might play a role in primary myxedema causing hypothyroidism and thyroid atrophy through inhibiting TSH-stimulated cAMP generation.