During a 6-month period, 100 patients with systemic lupus erythematosus (SLE) were consecutively studied for the presence of antithyroid antibodies and thyroid disease. Overall, the prevalence of antithyroid antibodies was similar in patients with SLE (21%) and controls (16%). However, antithyroglobulin antibodies were found in 11% of patients with SLE and only 2% of controls (p = 0.009). The levels of antimicrosomal antibodies were also different (median levels: SLE = 400; controls = 100) but this difference did not reach statistical significance. We found a significant correlation between thyroid stimulating hormone (TSH) levels and the levels of both the antithyroglobulin dilutions (p less than 0.05) and the antimicrosomal dilutions (p less than 0.05). This was not seen in the controls. A higher frequency of clinical thyroid disease was seen in patients with SLE with thyroid antibodies (5/21; 3 hypothyroid, 2 hyperthyroid) than in those without these antibodies (1/79; p = 0.001). Levels of antithyroid antibodies correlated with clinical or subclinical (marked by elevations of TSH) thyroid disease. Patients with SLE with these antibodies were significantly older (mean age 47.5 +/- 13 years) than those without antithyroid antibodies (mean age 37.5 +/- 12 years; p less than 0.001). Antithyroid antibodies define a subset of older patients with SLE with increased prevalence of both clinical and subclinical thyroid disease.