Although FSH has previously been found to be elevated during infancy in agonadal subjects, it is not known whether perinatal FSH levels are also increased. Neonatal blood spot FSH levels were studied retrospectively in nine full term girls born with Turner's syndrome and compared with presumably normal full term girls born the same week. FSH was measured using a highly specific immunoradiometric assay adapted to blood spots collected at the time of systematic neonatal screening. On day 5-6 after birth, FSH was undetectable (< 1 IU/L) or low (1-4.4 IU/L) in normal girls. Among the nine patients with Turner's syndrome, five had FSH levels below 3 IU/L, and four showed slightly elevated levels, ranging from 4.3-10.9 IU/L. These differences in FSH secretion were not related to differences in karyotype. Among five patients studied longitudinally during the first 6 weeks of life, three showed increases in FSH levels to 14.9-15.9 IU/L during the second week of life. However, this increase was comparable to that seen in some normal girls sampled on a second occasion during the first weeks after birth. One patient with Turner's syndrome still had low FSH (2.5 IU/L) on day 23, but showed some increase to 8.5 IU/L on day 30. We conclude that 1) in Turner patients, perinatal changes in FSH secretion are similar to those in normal girls, although there is already a lack of feedback control by gonadal hormones on the hypothalamo-pituitary axis; and 2) the FSH assay cannot be used for neonatal screening of Turner's syndrome.