Plasma concentrations of estrone and estradiol were determined by radioimmunoassay of cord blood from 17 neonates and of venous blood of 15 neonates, 61 boys and 69 girls of all developmental stages, 27 healthy women, 20 healthy men, and 89 male and female adolescents with different disorders of sexual development. In cord blood both estrone and estradiol averaged 15,000 pg/ml. Blood drawn from neonates shortly after birth showed levels between 300 and 500 pg/ml. These levels fell to 5-15 pg/ml within a few days after birth, which is the normal range for both sexes up to 7 years. Before puberty, estrone rises earlier in girls than in boys. The increase of estradiol follows the onset of puberty. Boys reach adult levels in stage IV, whereas girls in stage V still have lower estrogen levels than mature women. We did not find higher levels in 29 adolescents with gynecomastia than in normal adolescents at the same stage of puberty. We observed abnormally high estrogens in children with congenital adrenal hyperplasia, but, in spite of some very high levels, we did not observe premature thelarche or gynecomastia in children with this disease. Six females and 10 males with panhypopituitarism, aged from 10 to 19 years, revealed low values within the infantile range. Six boys between 5 and 14 years who suffered from anorchia had very low levels of estrogen, especially estradiol (which was nearly undetectable). In contrast to patients with functioning testicle tissue, the patients with anorchia did not show any increase of estrogens after stimulation with human chorionic gonadotropin. Abnormally low levels were found in females with Turner's syndrome, but only in those with 45, XO karyotype. Patients with 45, XO/46, XX, or 46, XXp~ karyotype had high levels of estrogen which corresponded to clinical signs of advanced puberty. Speculation As demonstrated by our results, the radioimmunologic method enables us to measure the low estrogen levels which are found in children. This will permit a more intensive examination of the pituitary-gonadal and pituitary-adrenal interrelation in the future.