Two brothers and their mother, who had allergic rhinitis caused by Japanese cedar pollen, developed silent thyroiditis in spring. In addition, these three patients had the same HLA haplotype. Another brother, the father, and the paternal grandmother, who did not have allergic rhinitis caused by Japanese cedar pollen, did not show symptoms of silent thyroiditis. The present study indicates that genetic and/or environmental factors are important in the development of this familial type of silent thyroiditis.