Infertility caused by ovarian failure is a characteristic feature in Turner's syndrome. Spontaneous pregnancies are seen in 2-5% of these women, and up to 30% have at least some pubertal development, indicating the presence of follicles in their ovaries in adolescence. It has not been clear at which age the follicles disappear. We analyzed the numbers and densities of follicles in ovarian cortical tissue from nine adolescent girls with Turner's syndrome who came to our clinics after having been informed about the study, with an aim to preserve ovarian tissue for possible infertility treatment later in life. A quarter to one whole ovary was laparoscopically removed for the procedure. Follicles were seen in the biopsy tissue in eight of nine subjects from whom ovarian tissue was laparoscopically obtained, the highest numbers being seen in the youngest girls and in those with mosaicism. In one 17-yr-old girl, no ovarian tissue was found. Follicle density was correlated with serum levels of FSH; individuals with the lowest FSH levels had the highest follicle density. One to 190 follicles were counted in the approximately 0.1-2.0 mm(3) of tissue analyzed, giving a density of 1.5-499 follicles/mm(3) of ovarian cortical tissue. Girls up to the age of 17 had primordial follicles in their ovaries. Three girls, two aged 15 yr and one aged 19, had only secondary follicles, with many being atretic. Our finding that adolescent girls with Turner's syndrome still have follicles in their ovarian cortical tissue raises the possibility of future fertility through cryopreservation of ovarian tissue. However, before such procedures can be recommended for clinical management, it is essential that future studies be performed to determine whether the oocytes retrieved from girls with Turner's syndrome have a normal chromosomal complement.