Gonadotropin-releasing hormone (GnRH) and olfactory neurons migrate together from the olfactory placode, and GnRH neurons eventually reside in the hypothalamus. Hypogonadism in male infants may be diagnosed in the first 6 months of life but cannot be diagnosed during childhood until puberty occurs. Patients with low serum testosterone and low serum gonadotropin levels have idiopathic hypogonadotropic hypogonadism (IHH). Mutations in three genes (KAL1, FGFR1, and GNRHR) comprise most of the known genetic causes of IHH. Treatment with testosterone is indicated if fertility is not desired, whereas GnRH or gonadotropin treatment induces spermatogenesis and fertility.