Eight cases of "triiodothyronine (T3) toxicosis" in Graves' disease have been identified. All eight patients had normal values for total or free thyroxine or both and significant elevations of T3. It is important for the physician to be aware of this entity and to suspect it in patients whose condition appears to be thyrotoxic but for whom standard laboratory tests reveal no abnormalities. The diagnosis must be based on laboratory findings, because these patients cannot be distinguished clinically from other patients with Graves' disease. In most instances, the condition is identified in patients with mild clinical toxic reaction, perhaps because the level of only one of the two metabolically active thyroid hormones is elevated. It is recognized that in patients with marked elevations of T3, severe thyrotoxicosis might be observed.