Partial thyroxine-binding globulin (TBG) deficiency in a family is described. A 43-year-old male was admitted because of the association of low thyroxine level but markedly elevated triiodothyronine resin-uptake despite his complaints of palpitation and excessive sweating. TBG was low (6 micrograms/ml) by radioimmunoassay. His free thyroxine level was normal. 123I uptake at 24 hours was normal and was suppressed following oral administration of triiodothyronine. Serum TSH level was normal and responded normally to TRH stimulation. Similar low level (6 micrograms/ml) in TBG was found in his elder brother who had no complaints. Another brother and 3 sisters had normal TBG levels. According to the classification of the TBG deficiency proposed by Barbosa et al., a family described here is considered to have the type II TBG deficiency which shows a much lower TBG level in male than in female with X-linked inheritance.