CONTEXT: The T1D Exchange includes a clinic-based registry, a patient-centric web site called Glu, and a biobank. OBJECTIVE: The aim of the study was to describe the T1D Exchange clinic registry and provide an overview of participant characteristics. DESIGN: Data obtained through participant completion of a questionnaire and chart extraction include diabetes history, management, and monitoring; general health; lifestyle; family history; socioeconomic factors; medications; acute and chronic diabetic complications; other medical conditions; and laboratory results. SETTING: Data were collected from 67 endocrinology centers throughout the United States. PATIENTS: We studied 25,833 adults and children with presumed autoimmune type 1 diabetes (T1D). RESULTS: Participants ranged in age from less than 1 to 93 yr, 50% were female, 82% were Caucasian, 50% used an insulin pump, 6% used continuous glucose monitoring, and 16% had a first-degree family member with T1D. Glycosylated hemoglobin at enrollment averaged 8.3% and was highest in 13 to 25 yr olds. The prevalence of renal disease was