Periodic paralysis has been known to be associated with thyrotoxicosis in Japan. The incidence was 8.6% among male and 0.4% among female thyrotoxic patients according to a survey performed in the three major thyroid clinics in Japan in 1957. To determine the changes in the incidence during the intervening 34 years, the same type of survey was carried out again in 1991 at the same three major thyroid clinics previously involved. The incidence of paralysis in 1991 was 4.3% among male and 0.04% among female thyrotoxic patients, indicating more than a 40% decrease in the incidence. The possible cause of the decrease is related to the changes in food consumption, namely, to the fact that less carbohydrate and more potassium were taken in 1991 than in 1957.