Graves' disease and Hashimoto's thyroiditis are common autoimmune thyroid disorders. Experimentally, 1,25(OH)(2) D(3) prevents Hashimoto's thyroiditis. Vitamin D serum levels in Graves' disease were found to be significantly lower than in nonautoimmune hyperthyroidism. The polymorphic vitamin D-binding protein (DBP) greatly facilitates vitamin D actions, and DBP alleles differ regarding their affinity for 1,25(OH)(2) D(3). Therefore, we investigated polymorphisms of the DBP gene for an association with thyroid autoimmunity. Families with an offspring affected by Graves' disease (95 pedigrees) or by Hashimoto's thyroiditis (92 pedigrees) encompassing 561 individuals of Caucasian origin were genotyped for three DBP polymorphisms [(TAAA)(N) in intron 8; StyI; and HaeIII in exon 11]. Indirect haplotyping and (extended) transmission disequilibrium testing were performed. There was a significant transmission disequilibrium of the intron 8 polymorphism in patients with Graves' disease (P < 0.03) but not of the exon 11 polymorphism. In contrast, neither the intron 8 nor the exon 11 polymorphism was associated with Hashimoto's thyroiditis. Maternal and paternal transmission as well as allele frequencies in DQ2(+) and DQ2(-) patients did not differ in either disease. Therefore, allelic variants of the DBP gene confer susceptibility to Graves' disease but not to Hashimoto's thyroiditis in our population. These findings support a role of the vitamin D endocrine system in thyroid autoimmunity.