A method of prediction of age of menarche from the height and weight of a girl at each age from 9 through 13 years using quartiles of total body water as percent of body weight, an indicator of body composition, particularly fatness, at each age is presented. A similar method to predict age of initiation of the adolescent growth spurt from the height and weight of a girl at age 8 years is also given. Comparison of the results with prediction by other methods supports the idea that the ratio of lean body weight to fat and the metabolic rate per kilogram are important for the determination of sexual maturation. The time difference between actual and predicted age of menarche was six months or less for 59% of the subjects at age 12, 53% at age 11, 48% at age 10, and 47% at age 9.