Stimulation with LH-RH (luteinizing hormone-releasing hormone) was performed in 29 children and adolescents with Turner's syndrome (15 less than 13 years old), and in 11 patients with Klinefelter's syndrome (7 before puberty). Synthetic LH-RH was injected intravenously in a dose of 25 mug/m2; plasma LH and FSH were determined radioimmunologically. The results show that patients with Turner's syndrome had pathologically elevated LH and FSH values at all ages. Normal LH and FSH results before and after LH-RH were found in 3 patients with X0/XX karyotype who had spontaneous menstruation. The 2 adult patients with Klinefelter's syndrome showed an exaggerated LH response and excessively high FSH levels before and after LH-RH, consistent with hypergonadotropic hypogonadism. Six out of 7 prepubertal boys with Klinefelter's syndrome, had a normal LH and FSH response to LH-RH. Only 1 prepubertal boy with cryptorchidism and genital maliformation in addition to the Klinefelter's syndrome had pathological values. The normal gonadotropin results in prepubertal patients with Klinefelter's syndrome show that their testes apparently loose the negative feedback activity on gonadotropin secretion only during puberty, at a time when tubular hyalinization appears. This is in contrast to Turner patients with gonadal dysgenesis who have hypergondadotropic hypogonadism already early in childhood.