Thirty-six cases of premature thelarche in girls 9 months to 7 years old have been studied. In most cases, breast development began before 3 years and came to regression. Onset after 3 years was less common and led usually to breast persistance. Height was lightly but significantly advanced. Plasma levels of estrone, estradiol (in 21 cases) and gonadotropins (FSH and LH, in 12 cases) were not significantly increased. However, LH-RH test (in 12 cases) suggested prolongation and/or increase of the physiologic gonadotropic hyperactivity of normal female infants.