Serum levels of immunoglobulins (Ig) IgA, IgG and IgM were measured in 96 HLA-typed patients with Graves' disease and an equal number of sex and age matched controls. Patients showed significantly increased serum immunoglobulin levels compared to controls. HLA DR3-positive patients had larger elevations of IgG concentration than their HLA-DR3-negative counterparts. Variation in serum immunoglobulin levels was not associated with sex, treatment or ophthalmopathy. It is suggested that the hypergammaglobulinemia of Graves' disease is related to defective suppressor cells triggered by Ig or Ig-linked genes. HLA-related gene(s) must, in addition, be involved to account for hypergammaglobulinemia G in the HLA DR3-positive subset of patients.