Seasonal variations in the occurrence of menarche in 1226 Japanese girls are not uniform, when they were divided into two groups by their season of births. Summer trough (May-July) was inapparent in girls born in summer (May-October), and winter trough (February-March) was less deep in girls born in winter (November-April). No differences were found in fall trough (September-November) between both groups. Girls born in summer (or winter) seem to be resistant to some summer (or winter) factors, which inhibit the occurence of menarche.