In a clinic population of 509 type 1 diabetic patients aged 16-45 years, 5.5% had received treatment for thyroid disorders (20 hypothyroid, three males; eight thyrotoxicosis, four males), and Addison's disease was present in four patients (0.8%, one male). In all patients, type 1 diabetes preceded the diagnosis of the other autoimmune disorder. The clinical presentation of hypothyroidism was usually insidious with few symptoms, although an increased frequency of hypoglycaemic symptoms and/or raised serum cholesterol levels often prompted thyroid function testing. In contrast, the patients with thyrotoxicosis had florid symptoms, weight loss (mean 8.12 kg), palpable goitres, increasing insulin requirements, and low cholesterol levels. Six patients did not achieve remission or had recurrent thyrotoxicosis after oral antithyroid treatment and required 131I or thyroid surgery. A family history of autoimmune disease was present in 25% of patients with thyroid disorders (seven thyrotoxic and one hypothyroid) and in three of the four patients with Addison's disease. In this population of young adult type 1 diabetic patients, appropriate tests for thyroid dysfunction and Addison's disease should be carried out if there is clinical suspicion and/or unexplained changes in diabetic metabolic control or serum cholesterol. Careful follow-up of patients with a family history of these conditions is recommended.