Kallmann syndrome is a rare genetic hormonal disorder which is estimated to affect between 1 in 10,000 and 1 in 50,000 people. It is thought to be between two and three times more common in men than women.
People with Kallmann syndrome have a hormonal deficiency which prevents the individual from starting or fully completing puberty. Kallmann syndrome is also accompanied by a lack of sense of smell, known as anosmia.

Kallmann syndrome is a form of congenital hypogonadotropic hypogonadism (CHH) which can also be be described as isolated GnRH deficiency.  It can occur with a normal sense of smell, it is only when the sense of smell is affected that the term Kallmann Syndrome is used.

Approximately 50% of cases of CHH are classed as Kallmann syndrome.  Without treatment, people with Kallmann Syndrome will remain sexually underdeveloped with poorly defined secondary sexual characteristics and almost invariably infertile.  Although most people have anosmia, a small minority have hyposmia, meaning that they have only a very weak sense of smell.  It is very difficult to diagnose Kallmann syndrome for certain until puberty is reached.

However, it can sometimes be detected earlier in childhood when a child cannot smell through genetic testing or through hormone testing during mini-puberty in infants. Kallmann syndrome is more easily diagnosed in a teenager or adult who has failed to go through puberty and in whom an abnormal sense of smell is observed, but many are too embarrassed about their lack of sexual development and are nervous about visiting their GP or primary care physician.  Consequently, there are probably many cases of Kallmann syndrome that go unreported.

More general information about congenital anosmia can be found here.