The mean age at menarche among 6,217 healthy American student nurses was 151.8 months. This age is lower than that reported in most previous studies and is 4.5 months lower than that of the mothers of these subjects. These data are consistent with the hypothesis that menarche is taking place earlier than it formerly took place. The sequence of overt maturational events which preceded menarche started with the appearance of pubic hair at 142.5 months and breast budding at 143.0 months, followed by the appearance of axillary hair at 144.9 months. Menarche occurred significantly earlier in the Northeastern region of the United States than elsewhere in the country. Menarche tended to occur prematurely in obese girls, except those weighing more than 30 per cent above normal. Paralytic poliomyelitis and migraine were associated with delayed menarche. A formula and nomograph are provided to estimate the 95 per cent confidence limits for the appearance of menarche in individual girls for whom the ages at breast budding and the appearance of pubic hair are known. © 1970.