Summary: Luteinizing hormone (LH), follicle-stimulating hormone (FSH) estrone, and estradiol were measured radioimmunologically in more than 60 plasma specimens of 34 girls suffering from idiopathic precocious puberty. Although the median values of both gonadotropins in the plasma of the patients (LH = 1.0 ng/ml; FSH = 1.6 ng/ml) were higher than those from healthy prepubertal girls (LH = 0.6 ng/ml; FSH = 0.5 ng/ml), only 23 out of 64 LH levels and 30 out of 64 FSH levels exceeded the upper normal limit for age. While the median value of plasma estrone (13 pg/ml) was found to lie within the normal range for prepubertal girls (7–29 pg/ml) and only 13 out of 75 estrone values were pathologically elevated, the median value of plasma estradiol (22 pg/ml) was nearly 3 times higher than the normal median for prepuberty (8 pg/ml). Of 75 estradiol levels, 35 were above normal for age. Grouping the values according to the stage of sexual development revealed considerably lower gonadotropin and estrogen levels in the patients than in normally maturing girls of the same developmental stage. However, patients who were examined repeatedly at short intervals over a 1-month period showed an almost cyclic sequence of their estradiol levels similar to the pattern observed in healthy pubertal girls. Speculation: The findings that plasma estrogens are relatively low in idiopathic precocious puberty in spite of well advanced sexual development suggest that the symptoms of this disturbance do not depend on estrogen concentrations alone. Other factors, such as enhanced receptor sensitivity or changes in the concentration of sexual hormone-binding globulin, must also be taken into consideration.