Genetic influences on variability of body weight and onset of menarche are well known. To investigate the genetic and environmental contributions to the association of body weight with onset of menarche, we studied Finnish twins from consecutive birth cohorts (the FinnTwin16 study) ascertained from the national population registry, which identifies nearly 100% of all living twins. Baseline questionnaires were mailed to the twins within 60 days of their sixteenth birthday and later to older sibs of the twins. Pairwise response rates (approximately 85% across gender and zygosity) and 30 months of data collection yielded results from 1283 twin pairs. The questionnaires included a survey of health habits and attitudes, a symptom checklist, MMPI personality scales, and a survey of relationships with parents, peers, and the co-twin. Age at menarche was reported by 468 monozygotic (MZ) girls, 378 girls from like-sex dizygotic (FDZ) pairs, 434 girls from opposite-sex (OSDZ) pairs, and 141 older female sibs of the twins. The one-month test-retest reliability of age at menarche in an independent sample (N = 136) of 16-year-olds from a national survey was 0.96. Girls from OSDZ pairs had a significantly higher mean age at menarche (13.33 yr) than FDZ girls (13.13 yr) (difference, 0.20 yr; 95% confidence interval, 0.05-0.35). The MZ correlation for age at menarche was 0.75, the DZ correlation was 0.31, and the sib-twin correlation was 0.32. A bivariate twin analysis of age at menarche and body mass index (wt/ht2) indicated that 37% of the variance in age at menarche can be attributed to additive genetic effects, 37% to dominance effects, and 26% to unique environmental effects. The correlation between additive genetic effects on age at menarche and body mass index was 0.57, indicating a substantial proportion of genetic effects in common.