This study was conducted to understand the clinical features and natural course of Taiwanese girls with premature thelarche (PT). The medical records of 91 Taiwanese girls with PT who were diagnosed younger than six and have been regularly followed up for more than two years were reviewed. For comparison, GnRH test was also done in 25 girls with central precocious puberty (CPP) and 10 normal prepubertal girls. The age of onset of these patients was 1.5 +/- 1.6 years and 79% of them developed PT before the age of two. Girls with PT had intermediate degree of hypothalamic-pituitary-ovarian activity between prepuberty and CPP with FSH-predominant response to GnRH stimulation. 87% of patients have complete regression of breast development during 3.8 +/- 2.5 years' follow-up but 19% of them have progressed to CPP during follow-up. We conclude that Taiwanese girls with PT more often developed within the first two years of life. Activation of hypothalamic-pituitary-gonadal axis with predominant FSH activity during infancy may contribute to its development. In addition, PT is not always a benign self-limited condition, and clinicians should be cautious about pubertal development of these patients.