A 17 year-old man, with periodic muscular weakness since the age of 6 years, is presented. The episodes of periodic paralysis were of variable duration, from 1 to 3 days, and were induced by physical exercise or by stress. Weakness was generalised, although predominant in anterior compartment of the legs, with foot drop. Interictal neurological examination was absolutely normal. He showed dysmorphic features, with micrognatia. Cardiac examination revealed continuous arrhythmia. Basal EKG and 24 hours EKG-Holter confirmed the existence of abundant ventricular extrasystoles, with episodes of ventricular tachycardia, without clinical manifestations. Echocardiogram was normal. Ictal and interictal ENG-EMG, and muscle and nerve biopsies were normal. Serum potassium levels during the episodes ranged from 3 to 3.6 mEq/l (N: 3.5-4.5 mEq/l), being normal interictally (4-5 mEq/l). Oral administration of potassium did not prevent the development of episodic weakness. He had no familial history of similar symptoms. This association of periodic paralysis, cardiac arrhythmia and dysmorphic features correspond to a rare entity named Andersen's syndrome.