Normal Japanese children are considered to have seasonal variation in body weight which increases in fall and winter and decreases in summer. However, abnormal seasonality was found in some of elementary school children in Tokyo, that is, body weight increased in summer in obese children. In most elementary schools in Tokyo, summer holidays begin in the latter part of July and end on August 31 . So, children might spend quite different life during the holidays compared with school days. The present study examined seasonality in body weight of 91 elementary school children living in Nagasaki City where the period of summer holidays is the same as in Tokyo. Then, the relationship between obesity and weight gain in summer holidays was discussed. Body weight has been measured monthly for six years (excluding August). Time-series analysis was made by the Program of Census Method II X-11 in which the original series O is assumed to be the product of three components as follows; O =C × S × I, where C is trend-and-cycle factor, S is seasonal factor, and I is irregular factor. Significant seasonal variations were observed in about 90% of the children. The pattern of seasonal variation of group means was low in summer and high from fall to winter or early spring in both sexes. Rohrer's index in 17 children were over 145 at least once through six years and they were classified as "obese group". Among the obese group 11 children had high seasonal factors from July to September, which indicates that body weight increased in summer holidays. In other obese children whose seasonal factors did not have peaks in September, Rohrer's index gradually lowered. Thus we conclude that weight gain in summer holidays is the most critical warning of obesity in school children.
