Two families in which more than two members had Alport's syndrome were examined. Serum-antithyroid-antibodies were found in all those with Alport's syndrome and some of their relatives. These included three cases of Alport's syndrome with nephritis and/or nerve deafness and two normal people who can be regarded as carriers of Alport's syndrome from one family, and one case of the syndrome with nephritis and deafness from the other family. One case of Alport's syndrome had symptoms of hypothyroidism, and biopsy specimens from another case were characteristic of chronic thyroiditis. The results indicate that serum-anti-thyroidantibodies, together with nephritis and deafness, are a sign of Alport's syndrome. The results accord with transmission of the syndrome as an autosomal dominant trait and suggest that Alport's syndrome may be caused by an immunological abnormality.