- Objective: To test the hypothesis that age at menarche is inversely related to body mass index (BMI). Population and Methods: A questionnaire was filled in by 11,293 young women at the time of discharge from compulsory military service, in order to determine the self-reported age at onset of menarche. The participants were weighed and measured and BMI was calculated. Obesity was defined as BMI >30 and leanness as BMI <20. Results: The age at menarche was significantly and inversely correlated with BMI (R = -0.11, p <0.001). The age at menarche of obese women (12.9 ′ 1.3 years) was significantly lower than that of non-obese, non-lean women (13.3 ′ 1.4 years), who themselves had a lower age at menarche than lean women (13.5 ′ 1.4 years) (p <0.001). Conclusions: We conclude from our data that the relationship between BMI and pubertal development in girls appears to be a continuum.