Background: Recent studies in the United States have demonstrated that a significant proportion of girls show thelarche before the age of eight years. Nutritional status, geographic influences and racial factors are known to affect the timing of puberty. Aim: To evaluate the age of onset of puberty, development of secondary sexual characteristics and menarche in Chilean girls, and its relation to obesity and socioeconomic status. Material and methods: Healthy girls attending elementary school, from first to ninth grade in Santiago, Chile, were studied. A pediatric endocrinologist evaluated pubertal development using Tanner classification. Breast development was assessed by inspection and breast palpation. Average age of onset of pubertal events was determined by probit analysis. Results: A total of 758 girls, aged 5.8 to 16.1 years, were recruited. Obesity, defined as a BMI greater than 90th percentile, was found in 24.4%. The age of menarche was 12.7 years, the onset of Tanner stage 2 breast development and pubic hair was at 8.9 and 10.4 years, respectively. Sixteen percent of girls aged 7 to 7,9 years, had thelarche. Upper class girls showed a later onset of breast Tanner stage 4 stage than low-middle class girls. Obesity was not found in logistic regression analysis to be a significant predictive factor in the onset of puberty. Conclusions: The age of menarche has not changed in the last thirty years, but an earlier onset of thelarche has occurred. The high frequency of thelarche between 7 and 8 years suggests that the normal age of breast development should be revised (Rev Méd Chile 2004; 132: 801-8)