The appearance of autoimmune thyroiditis in the course of other autoimmune diseases, which do not affect specific organs (systemic lupus erythematosus, Sjögren syndrome, rheumatoid arthritis and others), is more frequent than is usually believed. Nevertheless, it is scarcely studied, especially in children. The purpose of this study was to look for autoimmune lesions of the thyroid gland in children suffering from juvenile chronic arthritis (JCA) and systemic lupus erythematosus (SLE). Twenty seven children having JCA and twelve children with SLE, aged 5 to 18 years, were enrolled into study. In all of them the disease was in an active phase. The serum levels of total thyroid hormones (T3, T4) and TSH, thyroid antibodies (TAB and MAB) and antinuclear antibodies (ANAB) were analyzed using respectively fluoroimmunologic, microhemagglutination and indirect immunofluorescention tests. According to our results, autoimmune thyroiditis was found in 12 out of 27 children with JCA (44.4%); 85.2% of them were euthyroid, 11.1% had a compensated hypothyroidism, and 3.7% had Hashi-toxicosis. From a clinical point of view, very interesting was the combination of JCA, autoimmune thyroiditis and pseudoxanthoma elasticum in a 13-year old girl. Positive thyroglobulin antibodies (1:80-1:5120) were found in 17 out of 27 cases of JCA (63%). The microsomal antibodies were elevated (1:100-1:1600) in 7 out of 27 (25.9%); antinuclear antibodies (1:80-1:640) were detected in 15 out of 27 cases of JCA (55.5%). A simultaneous elevation of all three kinds of antibodies was found in 14.8% of children with JCA, and of TAB and MAB--in 18.5%. Thyroid gland disorders were detected also in children suffering from SLE. Thyroglobulin antibodies were positive (1:80-1:5120) in 7 out of 12 cases. Antinuclear antibodies (1:320-1:2560) were detected in 8 out of 12 cases (66.7%). The serum levels of T3, T4 and TSH were in the reference limits in all children with SLE. The present study suggests that involvement of the thyroid gland is not uncommon in autoimmune disease in Autoimmune thyroiditis can occur in association with other autoimmune diseases, affecting some organs or systems, such as the insulin-dependent diabetes mellitus, pernicious anaemia, thrombocytopenia, vitiligo, as well as some chromosomal aberrations--Turner's syndrome, Noonan's syndrome and Down's disease . The appearance of autoimmune thyroiditis together with other autoimmune diseases which do not affect specific organs (such as systemic lupus erythematosus, Sjögren syndrome) is the reason to classify them in a common subgroup of the autoimmune polyendocrine syndromes--type IIID . The rheumatic diseases are--more frequently than suspected--associated with autoimmune thyroiditis, but this connection is not well studied. The literature offers very scarce information on the problem, especially for the childhood. The purpose of this study was to look for autoimmune lesions of the thyroid gland in children suffering from juvenile chronic arthritis (JCA) and systemic lupus erythematosus (SLE).