Seven families with familial hypokalaemic periodic paralysis were found in Finland. Nine of the 103 asymptomatic family members studied had abnormal results on a potassium exercise test. The overall prevalence of familial hypokalaemic periodic paralysis in Finland was 0.4/100,000. Carbohydrate intake and hard exercise were the most important triggers of paralytic attacks. Half of the patients reported having attacks at least once a month. Seven patients reported cardiac symptoms (especially bradycardia) during attacks. Permanent muscular weakness was not prominent.