To ascertain the presence and significance of anticardiolipin antibodies in patients with Hashimoto's disease.We studied 19 patients (15 women and 4 men; age range, 26 to 79 years) with Hashimoto's disease. IgA, IgG, and IgM anticardiolipin antibodies were measured by enzyme-linked immunosorbent assay.Of the 19 study patients, 4 (21%) tested positive for anticardiolipin antibodies (IgG in 2 and IgM in 2). Patients with anticardiolipin antibodies did not exhibit a higher titer of antithyroglobulin or anti-thyroid peroxidase antibodies in comparison with patients without anticardiolipin antibodies. Moreover, no relationship was found between anticardiolipin antibody levels and thyroid hormone replacement therapy. None of the patients with high levels of IgG or IgM anticardiolipin antibodies had clinical features of the antiphospholipid syndrome.Our results showed an increased incidence of anticardiolipin antibodies in patients with Hashimoto's disease. The presence of these antibodies may be a nonspecific marker of activation of the immune system.