Objectives-To determine whether thyroid dysfunction is found with increased frequency in patients with rheumatoid arthritis (RA). Methods-A controlled prospective survey was conducted on a cohort of patients with RA derived from a hospital clinic and a private surburban rheumatology practice. A control group with similar demographic features was generated from the same sources and included subjects with either osteoarthritis or fibromyalgia. Consecutive patients were evaluated over a six month period. The evaluation included a complete history and physical examination, and determination of serum thyroxine, free thyroxine, triiodothyronine, thyroid stimulating hormone (IRMA), antinuclear antibodies, and rheumatoid factor. Results-Of the 91 women with RA evaluated, 29 (30%) had evidence of thyroid dysfunction compared with 10 (1 1%) of 93 controls. The excess thyroid dysfunction is due to either hypothyroidism or Hashimoto's thyroiditis and was independent of age, increasing duration of disease, rheumatoid factor, and antinuclear antibodies. Conclusions-Thyroid dysfunction is seen at least three times more often in women with RA than in women with similar demographic features with noninflammatory rheumatic diseases such as osteoarthritis and fibromyalgia.