Most cases of Type 1 diabetes mellitus (T1DM) are thought to result from selective autoimmune destruction of pancreatic beta cells. In keeping with the autoimmune aetiology, other autoimmune diseases occur with increased frequency. The purpose of this study was to determine the frequency of hypothyroidism, adrenocortical insufficiency, pernicious anaemia, coeliac disease, vitiligo and systemic autoimmune disorders in 504 patients with longstanding T1DM. The prevalence of documented hypothyroidism, adrenal insufficiency, pernicious anaemia, coeliac disease, vitiligo and systemic autoimmunity was found to be 20.2%, 1.8%, 0.4%, 0.6%, 2% and 1.6% respectively – consistently higher in females (p<0.0002 for hypothyroidism). Screening for thyroid (and possibly related) autoimmune disease should routinely be performed in newly diagnosed T1DM subjects, particularly female patients.