Early menarche and late menopause are risk factors for the development of breast cancer. An adequate hormonal mechanism for those observations has not been described. Because obesity has also been associated with increased breast cancer risk, we examined the relationship among height, weight, adiposity, and menstrual cycle characteristics in women who prospectively recorded menstrual cycle intervals and reproductive events as part of the Menstrual and Reproductive History Research Program. Within this study cohort, adult height, weight, and a derived index of adiposity were related to both earlier age at menarche and later age at menopause. The association of early age at menarche with adiposity persisted from age 18 yr through middle age and into the menopausal years. There was no relationship between body mass and age at first pregnancy, another breast cancer risk factor. Women who were heavier at 18 yr of age and thereafter gained 5 lb or more had significantly greater mean menstrual cycle length and variability during the 7-yr period after menarche than the other members of the cohort. There was no obvious relationship between weight indices and menstrual cycle pattern during mature reproductive life or during the 7 yr before menopause. Our data suggest that menarcheal and menopausal age may have a common relationship to nutritional status and to constitutional features that predate the onset of menarche and persist into later life.