Diabetes mellitus type 1 (Type 1 diabetes, Type I diabetes, T1D, T1DM, IDDM, juvenile diabetes) is a form of diabetes mellitus. Type 1 diabetes is an autoimmune disease that results in the permanent destruction of insulin producing beta cells of the pancreas. Type 1 is lethal unless treatment with exogenous insulin via injections replaces the missing hormone.

Type 1 diabetes (formerly known as "childhood", "juvenile" or "insulin-dependent" diabetes) is not exclusively a childhood problem: the adult incidence of Type 1 is noteworthy — many adults who contract Type 1 diabetes are misdiagnosed with Type 2 due to the misconception of Type 1 as a disease of children — and since there is no cure, Type 1 diabetic children will grow up to be Type 1 diabetic adults.

There is currently no preventive measure that can be taken against type 1 diabetes. Most people affected by type 1 diabetes are otherwise healthy and of a healthy weight when onset occurs, but they can lose weight quickly and dangerously, if not diagnosed in a relatively short amount of time. Diet and exercise cannot reverse or prevent type 1 diabetes. Although there are clinical trials ongoing that aim to find methods of preventing or slowing its development, so far none has proven successful.

The most useful laboratory test to distinguish Type 1 from Type 2 diabetes is the C-peptide assay, which is a measure of endogenous insulin production since external insulin (to date) has included no C-peptide. However, C-peptide is not absent in Type 1 diabetes until insulin production has fully ceased, which may take months . The presence of anti-islet antibodies (to Glutamic Acid Decarboxylase, Insulinoma Associated Peptide-2 or insulin), or lack of insulin resistance, determined by a glucose tolerance test, would also be suggestive of Type 1. As opposed to that, many Type 2 diabetics still produce some insulin internally, and all have some degree of insulin resistance.

Testing for GAD 65 antibodies has been proposed as an improved test for differentiating between Type 1 and Type 2 diabetes.