While most of the children spontaneously recover menstruation or experienced normal puberty after chemotherapy, their ovarian reserve may be impaired by treatment inducing future infertility. Fertility preservation is currently proposed for selected prepubertal patients with a high risk of premature ovarian failure after treatment (mostly conditioning regimen for bone marrow transplantation). For patients with low or moderate risks, counselling is very difficult and no fertility preservation procedure is usually proposed for these patients as no marker of the ovarian reserve has been validated in this young population to assess the individual risk. The primary objective of the study is to prevent long-term treatment-related infertility by detecting the young patients who normally progressed to menarche but have a reduced ovarian reserve. These patients may benefit from particular follow-up and fertility preservation procedure.