A male aged 21 was found to have elevated serum PBI on several occasions. Further studies revealed clinical euthyroidism with raised serum concentrations of T4, T3 and free T4 with normal hormone binding capacity of the thyroxine binding proteins and normal thyroid-pituitary relationships. The response to TRH was high normal or on one occasion exaggerated. The extrathyroidal thyroxine pool and the daily turnover of thyroxine were considerably above the upper limit of the normal. The subject had no goitre. These findings indicated partial peripheral resistance to thyroid hormones including the pituitary gland. There were no circulating antibodies to thyroglobulin, thyroid microsomes, T4 or T3. A study of the propositus' family revealed an autosomal dominant heredity of this abnormality. Similar findings were found in the brother, mother and the two uncles of the propositus. In these subjects the serum concentrations of T4, T3 and free T4 as well as that of rT3 were raised above the normal. The subjects studied were clinically euthyroid. Goitre was present only in the older members of the family. The response to TRH was usually high normal or at times exaggerated and the basal TSH level seemed to fluctuate during years of follow-up indicating a continuous adjustment of the thyroid function by means of TSH secretion. The mother and the older uncle had previously been treated because of hyperthyroidism. Only the mother had circulating thyroid antibodies and none of the family members had antibodies against T4 or T3. This hereditary condition seems to have a very mild clinical manifestation in this family.