In the National Child Development Study (1958 cohort) information on their age at menarche and their weights and heights measured at 7, 11, and 16 years was available for 4427 girls. The distribution of age at menarche was not influenced by social class. Weight adjusted for height did not play an important part in the timing of sexual maturation of the girls in the study. Relative weight (weight expressed as a percentage of standard weight) at the ages of 7 and 11 years explained only 3.2%, and 4.9%, respectively of the variation in age at menarche, and changes in relative weight between these two ages accounted for 2%. Girls with early menarche were more likely to be overweight at ages 7, 11, and 16 years than those with late menarche, although early menarche was also reported by girls who were underweight or of average weight. These findings support the hypothesis that in well nourished populations the relation between menarche and body size is largely regulated by genetic factors and that nutrition is less important.