Turner's syndrome is associated with autoimmune disorders. Autoimmune endocrinopathy in Turner's syndrome seems to be limited to autoimmune thyroiditis. A small number of patients with Turner's syndrome has also been associated with celiac disease, inflammatory bowel disease and juvenile rheumatoid arthritis. Type 1 diabetes mellitus in Turner's syndrome has been rarely reported. We present here the youngest patient with Turner's syndrome who developed type 1 diabetes mellitus. At the age of 3.5 years she was hospitalized with diabetic ketoacidosis. Anti-islet cell and anti-insulin antibodies were positive and C-peptide level was low. When she was investigated for recurrent urinary tract infections, horseshoe kidney was detected by ultrasonography. Karyotype analysis revealed 45,XO. She has been followed for 2 years with an insulin dose of 0.9 U/kg per day. The prevalence of type 1 diabetes mellitus associated with Turner's syndrome is still unknown.