X-linked ichthyosis is a genetic disorder of keratinization characterized by a generalized desquamation of large, adherent, dark brown scales. Extracutaneous manifestations include corneal opacity and cryptorchidism. Since 1978 it has been known that a deficit in steroid sulphatase enzyme (STS) is responsible for the abnormal cutaneous scaling, although the exact physiological mechanism remains uncertain. The STS gene has been mapped to the distal part of the short arm of the X chromosome. Interestingly, this region escapes X chromosome inactivation and has the highest ratio of chromosomal deletions among all genetic disorders, complete deletions having been found in up to 90% of patients. Diagnosis of patients with X-linked ichthyosis and female carriers is based on biochemical and genetic analysis. The latter currently seems to be the most accurate method in the majority of cases.