Pendred's syndrome, an autosomal-recessive condition characterized by congenital sensorineural hearing loss and goiter, is caused by mutations in the PDS gene. Located on chromosome 7q22-q31, it encodes a chloride-iodide transporter expressed in the thyroid, inner ear, and kidney. We investigated the PDS gene of six affected individuals from four unrelated families with Pendred's syndrome by direct sequencing. PDS mutations were identified in homozygous or compound heterozygous state in all six cases. A homozygous missense mutation leading to the amino acid substitution S133T was detected in a family of Turkish origin. The mutations found in the other affected individuals, who originate from Germany, were V138F/Y530H, V138F/E384G, and V138F/V138F. Because V138F was found in the German patients with Pendred's syndrome on at least one allele, we genotyped five microsatellite markers located in the PDS region. All affected German individuals shared a common haplotype at three microsatellite markers located close to or within the PDS gene. We therefore concluded that V138F is a founder mutation in our cohort of German families with Pendred's syndrome.