introduction: type 1 diabetes (dm1) is frequently associated with other immune diseases, especially autoimmune thyroid disease (atd). objective: to determine the prevalence of atd in subjects with dm1 and to investigate possible association with other factors. methods: tsh, free t4, antiperoxydase antibody, and hemoglobin a1c were measured in 126 dm1 patients. results: atd was found in 26 (20.6%) patients being 11 (8.7%) with overt hypothyroidism, 6 (4.8%) with subclinical hypothyroidism and 9 (7.1%) with thyroiditis without thyroid dysfunction. in one patient (0.8%), atd (hyperthyroidism) preceded dm1. atd was associated with current age and age at diagnosis of dm1. no associations of atd with sex, race, duration of dm1, parity, and home place were observed. conclusions: the prevalence of atd in dm1 patients is high enough to justify screening and is related to current age and to the age at the diagnosis of dm1.