Two cases of recurrent monoarthritis are described in which antithyroid microsomal (antiMi) autoantibody (Ab) was found in synovial fluid (SF) before any clinical or serological evidence of thyroid disease. Subsequently, the follow-up of the two patients showed the appearance of thyroiditis within 2-5 years. The presence of anti-Mi Ab in SF might anticipate the appearance of autoimmune thyroiditis even in the absence of serum detectable antithyroid Ab, as was later observed in these two cases.