This study presents clinical, audiological, radiological and biochemical data on 14 individuals representing 6 families, each including at least one patient with the classical Pendred's syndrome. The size of the thyroid, degree of hearing loss and result of the iodine perchlorate discharge test showed great variations, even between affected relatives. Based on these observations a new definition of Pendred's syndrome is proposed. The use of the iodine perchlorate discharge test for detection of healthy carriers among family members is suggested.