The frequent occurrence of both rheumatoid arthritis and autoimmune thyroiditis was already investigated with in part many conflicting results. We investigated a number of 792 patients (383 of them suffering from rheumatoid arthritis and 409 with osteoarthritis). In all patients antithyroid peroxidase and antithyroglobulin antibodies were determined. Patients with rheumatoid arthritis showed a significantly higher occurrence of circulating thyroid antibodies than those with osteoarthritis (9.1% versus 3.7%, p = 0.0016). We conclude that there exists a cumulate coincidence of both diseases. Patients suffering from rheumatoid arthritis should undergo a thyroid examination especially for the presence of autoimmune thyroiditis.