Kallmann syndrome (KS) is a genetic disorder that prevents a person from starting or fully completing puberty. Kallmann syndrome is a form of a group of conditions termed hypogonadotropic hypogonadism. To distinguish it from other forms of hypogonadotropic hypogonadism, Kallmann syndrome has the additional symptom of a total lack of sense of smell (anosmia) or a reduced sense of smell. If left untreated, people will have poorly defined secondary sexual characteristics, show signs of hypogonadism, almost invariably are infertile and are at increased risk of developing osteoporosis. A range of other physical symptoms affecting the face, hands and skeletal system can also occur. The underlying cause is a failure in the correct production or activity of gonadotropin-releasing hormone by the hypothalamus. This results in low levels of the sex hormones testosterone in males or oestrogen and progesterone in females. Diagnosis normally occurs during teenage years when puberty fails to start. Lifelong treatment for both sexes is normally required. Hormone replacement therapy (HRT) is the major form of treatment with the aim to replace the missing testosterone or oestrogen and progesterone. Specialised fertility treatments are also available. The condition is more commonly diagnosed in males than in females. A 2011 study of the Finnish population produced an estimated incidence of 1 in 48,000 people overall, with 1 in 30,000 for males and 1 in 125,000 for females. Kallmann syndrome was first described by name in a paper published in 1944 by Franz Josef Kallmann, a German-American geneticist. The link between anosmia and hypogonadism had already been noted by Spanish doctor Aureliano Maestre de San Juan in 1856. Signs and symptoms It is normally difficult to distinguish a case of Kallmann syndrome (KS)/hypogonadotropic hypogonadism (HH) from a straightforward constitutional delay of puberty. However, if puberty has not started by either age 14 (girls) or 15 (boys) years and one or more of the non-reproductive features mentioned below is present, then a referral to reproductive endocrinologist might be advisable. The features of KS and other forms of HH can be split into two different categories; "reproductive" and "non-reproductive". Reproductive features Failure to start or fully complete puberty. Lack of testicle development in men (size < 4 ml, whereas the normal range is between 12 and 25 ml). Primary amenorrhoea (failure to start menstruation). Poorly defined secondary sexual characteristics. Micropenis in 5-10% of male cases. Cryptorchidism (undescended testicles) at birth. Low levels of the