OBJECTIVE To determine whether the titer and avidity of the thyroid peroxidase antibody differs between pregnant women in their first trimester who have a history of recurrent miscarriage and whose pregnancies continue to term and those whose pregnancies fail again later in the first trimester. METHODS Controlled clinical study. METHODS Healthy volunteers in an academic research environment. METHODS Pregnant women in their first trimester who had a history of recurrent miscarriage (> or = 3 miscarriages) and who were known to be positive for the thyroid peroxidase antibody. METHODS None of the patients received any medication. METHODS Thyroid peroxidase antibody titer and avidity (i.e., the net binding strength between antibody and antigen). RESULTS At the time of presentation, thyroid peroxidase antibody titer and avidity was significantly higher in those women who later miscarried compared with those whose pregnancies continued. In those whose pregnancies continued to term, titer and avidity declined as the pregnancy progressed. CONCLUSIONS Autoimmunity plays a role in recurrent miscarriage. Among a group of patients who had had recurrent miscarriages, there appeared to be differences in the humoral response to the pregnancy between those whose pregnancies continued to term and those whose pregnancies failed again.