Secular trends toward a declining age at puberty onset with correlated changes in body weight have been reported in economically advanced countries. This has been attributed to excess calorie intake along with reduced physical activity in children. However, because the timing of puberty in humans is also influenced by other factors, such as genetic traits, living conditions, geographical location, and environmental chemicals, it is difficult to distinguish the effect of diet and body size from other factors in a human population. Here we report that feeding juvenile female rhesus monkeys born and raised at the Wisconsin National Primate Research Center with a high-calorie diet results in acceleration of body growth and precocious menarche. The monkeys fed a high-calorie diet also had an elevated body mass index. The most significant treatment effects on circulating hormones were increased leptin and IGF-I levels throughout the experiment. The findings of this study suggest the importance of close monitoring of juvenile feeding behaviors as an important intervention to reduce the prevalence of precocious development and metabolic diseases in adulthood.