There has been controversy over the possible association of autoimmune thyroiditis with systemic lupus erythematosus (SLE). We have therefore compared the prevalence of thyroid autoantibodies and abnormal thyroid-stimulating hormone (TSH) levels in 41 SLE patients with age- and sex-matched controls. Twenty-one (51%) of the SLE patients had thyroid antibodies compared to 11 (27%) controls (p less than 0.05). Ten SLE patients and five controls had elevated TSH levels, usually in association with the presence of thyroid autoantibodies. These results suggest that SLE is associated with autoimmune thyroiditis and that many patients with SLE have minor hypothyroidism.