Pendred's syndrome is an autosomal recessive disease characterized by goiter and congenital sensorineural deafness. Most patients with Pendred's syndrome are euthyroid, but the perchlorate test is positive indicating an impaired iodide organification. The sensorineural deafness is typically associated with a malformation of the inner ear, referred to as Mondini cochlea. The incidence of Pendred's syndrome is thought to be as high as 7.5 to 10 in 100,000 individuals, and it has been estimated to account for about 10% of the cases with hereditary deafness. Linkage of Pendred's syndrome to chromosome 7q22-31.1 was first established in 1996, and the Pendred's syndrome gene (PDS gene) was cloned in 1997. The PDS gene encodes pendrin, a highly hydrophobic 780 aminoacid protein with 11 transmembrane domains. Its function is unknown. Sequence comparison reveals a very high homology to several sulfate transporters suggesting that it could be a sulfate or anion transporter. A wide spectrum of mutations in the PDS gene has now been associated with Pendred's syndrome. Molecular analysis of the PDS gene is useful to make a definite diagnosis in familial and sporadic cases with Pendred's syndrome, and will be helpful for determining the true prevalence of this disorder.