Previous data have indicated that organ-specific and non-organ-specific autoimmune diseases may occur in the same patient. We report here our study on the type and prevalence of endocrine autoimmune diseases in undifferentiated connective tissue disease (UCTD). A retrospective analysis revealed five out of 75 UCTD cases (6.6%) with cytology-verified autoimmune thyroiditis (associated with insulin-dependent diabetes mellitus in one case). Other UCTD patients had Graves' disease (one case), non-toxic multinodular goitre (two cases) and central hypothyroidism (one case). In a prospective study, thyroid function was evaluated in 15 consecutive UCTD patients with neither clinical nor laboratory signs of thyroid involvement. Basal and post-TRH stimulation TSH levels were significantly higher in UCTD patients than in healthy subjects.