We typed coded sera from 135 healthy controls, seventy-six patients with autoimmune goitrous and seventy-three with atrophic thyroiditis for IgG heavy chain markers (Gm). All subjects were Caucasian from Newfoundland. An increase in the Gm phenotype ag was found in the 149 patients with thyroiditis compared to controls (chi 1(2) = 5.82, P less than 0.01); significance was, however, not maintained after correction for the number of variables tested. The difference in ag phenotype was more pronounced among the seventy-three patients with atrophic thyroiditis (chi 1(2) = 8.80 corrected P less than 0.05). Because the haplotype ag was not significantly increase in this group, we conclude that homozygotes for Gm ag are at an increased risk of developing atrophic thyroiditis.