Little is known about the impact of age at menarche on later mortality. In a cohort of 61,319 Norwegian women interviewed in 1956-1959, the authors analyzed associations between age at menarche and all-cause mortality. A total of 36,114 women died during the 37 years of follow-up. An inverse association was found between age at menarche and the all-cause mortality rate (p < 0.001), with an approximately 2.4% (95% confidence interval: 1.6, 3.1) reduced mortality per year increase in age at menarche. The association was stronger in women with an attained age of less than 70 years (3.9% reduction in mortality) than in women aged 80 years and above (1.5%). The inverse association could not be explained by extreme mortality rates in women with very early (10 years) or late (19 years) menarche or by possible confounding variables such as birth cohort, place of residence, occupational category (own or husband's occupation), body mass index, age at first delivery, or parity. Because of lack of data, residual confounding by physical activity or cigarette smoking could not be ruled out. Women with a menarche at age 18 years or later had, however, a slightly higher mortality rate than was predicted by the linear association.