The prevalence of thyroid abnormalities was determined in 131 patients: group I = rheumatoid arthritis, 68 patients, group II = systemic immunological diseases (IIa Sjögren's syndrome, n = 6; IIb other rheumatic autoimmune disease, n = 17), group III = other rheumatic diseases n = 13 and control group (n = 27). Thyroid abnormalities (hypo, hyperthyroidism, nodular goiter) were frequent: 33.8% in group I, 100% in group IIa, 11.7% in group IIb. Hypothyroidism was present in 19.1% (group I), 50% (group IIa). 6% (group IIb). Autoimmune thyroiditis was found in 16.2% in group I, 100% in group IIa, 11.7% in group III. Thyroid diseases are frequent in patients with rheumatoid arthritis. Therefore thyroid tests might be performed in patients with rheumatoid arthritis.