Hypokaliaemic periodic paralysis is a rare hereditary neuro-muscular disease caused by an error in the ion-canals in muscle cells resulting in decreased excitabiliy. It presents itself in the late childhood or teenage years with a periodic paralysis without involving respiratory and heart muscles. Our patient was a 13-year-old boy, who woke up with decreased strength in arms and legs after excessive physical activity and a high carbo-hydrate intake. Tests showed a low P-potassium level. The patient had full remission of his symptoms within 24 hours.