OBJECTIVE To study the influence of birthweight, and weight and height at age seven years, on menarcheal age in a national sample of 1471 girls in England, Scotland and Wales. METHODS We studied 1471 girls included in the MRC National Survey of Health and Development. During medical examinations carried out by school doctors in this cohort, born in the first week of March 1946, the mothers of girls were asked whether their daughters had started to menstruate, and if so, the month and year when this happened. Anthropometric measurements at birth and at age seven years were also obtained. RESULTS Girls who were heavier at age seven years had menarche at an earlier age. The average age at menarche of those in the highest fifth of the distribution of weight at seven years was 7.3 months less than that of those in the lowest fifth of the distribution. In contrast, girls who were heavier at birth had menarche at a later age. The average age at menarche of those in the highest fifth of the birthweight distribution was 2.2 months more than those in the lowest fifth. These opposing trends of birthweight and weight at seven years on age at menarche were observed across the distribution of each variable, and exerted statistically significant (P < 0.001) independent effects in a multivariate model. CONCLUSIONS These observations are consistent with the hypothesis that menarcheal age is linked to programmed patterns of gonadotrophin release established in utero, when the fetal hypothalamus is imprinted, and is subsequently modified by weight gain in childhood.