The serum free thyroxine concentration was measured by direct radioimmunoassay in 38 untreated T3-thyrotoxic patients with elevated serum total and free triiodothyronine, normal serum thyroxine and free thyroxine index, no TSH response to TRH, and with clinical evidence of hyperthyroidism. An elevation of circulating free thyroxine values was observed in 58% of the patients, whereas total serum thyroxine concentration was within the normal range. It is suggested, therefore, that T3-thyrotoxicosis should be reserved for patients with elevated serum total T3 and free T3 concentrations and normal serum total T4 and free T4 concentrations. Serum thyroxine-binding globulin concentrations were significantly lower (P less than 0.025) in patients with an elevated serum free thyroxine (18.7 +/- 3.6 micrograms/ml: mean +/- SD) as compared with those in patients with a normal free thyroxine concentration (23.4 +/- 2.6 micrograms/ml). In addition, no daily fluctuations in total and free thyroxine concentration were observed in 6 patients over a 4-8 day period.