Bidirectional tachycardia is an uncommon arrhythmia that usually occurs in aged persons with severe myocardial disease or digitalis intoxication, and carries a poor prognosis. This is a report of a young woman with familial hypokalemic periodic paralysis, who has a 13-year history of asymptomatic bidirectional tachycardia in the absence of organic heart disease or digitalis intoxication. Association of periodic paralysis and bidirectional tachycardia in this case and four previously reported cases suggests a strong relationship between this arrhythmia and potassium.