Continuous plasmapheresis with the IBM blood cell separator showed to be a successful therapy in a 26 year old patient with thyroid storm. The patient regained consciousness already during plasmapheresis whereby 5 liters of plasma were exchanged within 3.5 h. Although 633 mug thyroxine (T4) and 13.6 mug triiodothyronine (T3) were removed with the 51 plasma, no fall occurred in total serum T4 and T3 levels, probably due to the rapid reflux of thyroid hormones from the tissue into the intravascular space. Though total T4 and T3 did not change, there was a significant fall in free thyroid hormone levels documented by reduction of T3-uptake test and particularily of urine T4 and T3 excretion. These findings show that the elevation of thyroid hormone binding sites with donor plasma is even more important than the removal of thyroid hormones.