A 31-year old cook suffered from episodes with muscular weakness since 3 weeks. These episodes resolved, at times, spontaneously and completely after a few hours. The weakness affected mainly the muscles of trunk and proximal extremities. Investigation of the patient revealed flaccid tetraparesis with diminished tendon reflexes, however, cranial muscles were not affected. Mental function and sensibility were intact. These findings were consistent with hypopotassemic periodic paralysis rather than a neurological or psychiatric disease. Serum potassium was determined as 1.8 mmol/l. Moreover, the patient suffered from atrial fibrillation. Grave's disease could be identified as the underlying cause of hypopotasemic periodic paralysis and atrial fibrillation. Further manifestations of primary hyperthyroidism were nervousness, diarrhea and weight loss of 5 kg observed during the previous months. The symptoms resolved under treatment with potassium, betablocker and thyrostatic agents. The patient was treated with warfarin because of atrial fibrillation and finally underwent electroconversion.