BACKGROUND AND AIMS: Retrospective studies have indicated that internationally adopted girls are at high risk of developing precocious puberty. Hypothetically, this could be due to selection bias. The aim of this study was to determine age at reaching pubertal milestones in healthy internationally adopted girls in a prospective, clinical study. METHODS: A longitudinal cohort study including 276 randomly recruited internationally adopted girls. At baseline, age ranged from 4 to 13 years. Participants were followed with biannual examinations over a period of 2 years. Examinations included height, weight, Tanner staging, blood sampling and bone age assessment. Age distribution at entering pubertal stages B2-B5 (breast development), PH1-PH5 (pubic hair development) and menarche was estimated by probit analysis. Data were compared to a reference population of Danish-born girls, studied cross-sectionally. RESULTS: Mean age at B2+ was 9.5 years (95% prediction interval 7.1-12.0 years) and mean age at menarche was 12.1 (10.2-14.0) years in adopted girls, which was significantly lower compared to the reference group (p < 0.0003). 16% of adopted girls entered stage B2 before 8 years of age. The puberty-related rise in LH, FSH and estradiol was detected at earlier ages in adopted girls compared to the reference group. CONCLUSION: Internationally adopted girls have a significantly higher risk of precocious pubertal maturation compared to Danish-born girls.