Hypolalemic periodic paralysis is an uncommon complication of thyrotoxicosis. It usually affects men of Oriental origin and presents with acute and profound muscle weakness especially of the lower extremities. We report a 46 year old man from Kuwait with sudden paralysis of both legs. Physical examination showed signs of thyrotoxicosis and laboratory tests revealed severe hypokalemia, very low serum thyroid-stimulating hormone and very high free thyroxin. The motor deficit regressed within 8 hours of 80 mmol ofKCI infusion. Awareness of such a problem is essential to avoid life threatening cardiac arrhythmia. Once recognized the treatment is simple and the prognosis is excellent.