Adult-onset Type 1 diabetes is simply Type 1 diabetes with new-onset at about age 20 or older. Most cases of Type 1 diabetes are autoimmune in nature, resulting from immune-mediated destruction of the beta cells of the pancreas. Markers for the immune-mediated destruction of the beta cells include autoantibody markers glutamic acid decarboxylase autoantibodies (GAD), islet cell cytoplasmic autoantibodies (ICA), insulinoma-associated 2 autoantibodies (IA-2), insulin autoantibodies (IAA), and zinc transporter 8 autoantibodies (ZnT8). Other autoantibodies may exist but have not yet been discovered. Type 1 diabetes is associated with HLA DR3/4 genes. People with Type 1 diabetes require exogenous insulin for survival. Type 1 diabetes was previously called juvenile diabetes, and the myth that Type 1 diabetes is a childhood disease creates significant problems for those diagnosed as adults. The truth is that Type 1 diabetes can be diagnosed at any age and adult-onset Type 1 diabetes far outnumbers childhood-onset Type 1 diabetes.
Adults with new-onset Type 1 diabetes frequently experience the problem of incorrect or delayed diagnosis which results in needless human suffering and can result in the rapid onset of diabetic complications and even death due to diabetic ketoacidosis (DKA). New-onset Type 1 diabetes in adulthood is far more prevalent than childhood-onset Type 1 diabetes, but the myth that Type 1 diabetes is a childhood disease and the rising epidemic of Type 2 diabetes often means that adults with new-onset Type 1 diabetes are incorrectly diagnosed as having Type 2 diabetes, an altogether different disease. Patients who are misdiagnosed and treated as if the patient has Type 2 diabetes are often undertreated with medications that do not help their medical condition. As stated in a Wall Street Journal article on the problem of misdiagnosis (“Wrong Call: The Trouble Diagnosing Diabetes,” August 2012), "Most of my [adult Type 1 patients] have been misdiagnosed as having Type 2," says Robin Goland, co-director of the Naomi Berrie Diabetes Center at Columbia University Medical Center in New York. "Once the right diagnosis is made the patient feels much, much better, but they are distrustful of doctors and who could blame them?"
In 2016, the American Association of Clinical Endocrinologists (AACE) stated that Type 1 diabetes should be confirmed by the presence of autoantibodies, to distinguish between T1D and T2D and determine appropriate treatment. Dr. David Bell suggests that autoantibody testing assists with identifying the type of diabetes, guiding the clinician to appropriate treatment with insulin, avoiding a period of poor glycemic control when oral therapy [medications for Type 2 diabetes] is failing, optimizing the potential for preservation of beta cell function, and maximizing the prevention of long-term complications of diabetes.
Well-known people who were diagnosed with Type 1 diabetes as adults include actress Mary Tyler Moore (diagnosed at age 33), former acting Surgeon General Dr. Kenneth Moritsugu (age 49), UK Prime Minister Theresa May (age 56), and Olympic swimmer (10 Olympic medals) Gary Hall, Jr. (age 24).
 Type 1 diabetes in adults can be acute onset or slowly progressive. Slowly progressive Type 1 diabetes is sometimes called latent autoimmune diabetes in adults (LADA) or Type 1.5 diabetes. More recently, diabetes researchers have discouraged the use of the term latent autoimmune diabetes in adults, because LADA is not a latent disease.
 The U.S. Centers for Disease Control and Prevention’s (CDC’s) most current information on the prevalence and incidence of Type 1 diabetes comes from Diabetes in America, Chapter 3, “Prevalence and Incidence of Insulin-Dependent Diabetes” (Diabetes in America, Second Edition, 1995). Although people who use that reference as a source of incidence statistics state that there are about 30,000 new cases of Type 1 diabetes each year and that half of those cases are children; in fact, that source states that children (<20 years of age) account for 13,171 cases and adults (>20 years of age) account for 16,542 cases, for a total of 29,713 new cases of Type 1 diabetes per year, 56% seen in adults. Furthermore, that source states that there is an unknown number of adults identified as having Type 2 diabetes who have slowly progressive Type 1 diabetes (Michael J. Haller MD, in Type 1 Diabetes Sourcebook (ADA/JDRF, 2013), says “Importantly, adults with LADA may represent an additional 10% of those adults incorrectly diagnosed with Type 2 diabetes.”) The number of people with slowly progressive Type 1 diabetes is quite large, consistently about 10% of “Type 2” diabetes based on autoantibody testing (for example, the first study that demonstrated that about 10% of people with “Type 2” diabetes are autoantibody positive was published in The Lancet in 1977. In the UKPDS, about 10% of people diagnosed with “Type 2” diabetes were autoantibody positive and had been misdiagnosed). Clinicians use autoantibody tests to distinguish between Type 1 autoimmune diabetes and non-autoimmune diabetes (Type 2 diabetes, monogenic diabetes, etc.).