Breast development in a girl 3 years of age or younger is a commonly encountered scenario. Nearly all of these cases will either regress or fail to progress during follow-up, confirming a diagnosis of premature thelarche (PT). Studies show that these girls will have onset of true puberty and menses at a normal age. The authors present evidence that laboratory testing, particularly basal and gonadotropin hormone-releasing hormone –stimulated gonadotropin levels, will show overlap between girls with PT and the rare patients with the onset of central precocious puberty before age 3, mainly of whom have hypothalamic hamartomas.