A 25-year-old Caucasian man is admitted to hospital because of severe hyperthyroidism. Soon after his admission, he develops a lower limbs proximal muscles paresis with tendinous hyporeflexia. He has presented similar episodes in the previous months. Serum potassium level is 2.9 meq/l whereas it was 4.1 meq/l a few hours before. The patient receives intravenous potassium and propranolol. He will not develop a similar episode afterwards. Hypokalemic thyrotoxic periodic paralysis associated with Basedow's disease is diagnosed. Hypokalemic thyrotoxic periodic paralysis is a very rare cause of intermittent muscular weakness in the young Caucasian male. The frequency of this entity is at least ten times higher in Asiatic hyperthyroid people. Its physiopathology involves intracellular potassium shifts in which Na/K ATP-ases of cell membranes, the number and the activity of which increase in hyperthyroid people, seem to play a major role. Treatment consists of correction of hyperthyroidism and administration of potassium during the acute episode Propranolol is efficient in preventing recurrent episodes of paralysis. Points of comparison between hypokalemic thyrotoxic periodic paralysis and hypokalemic familial periodic paralysis are discussed.