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 (Many scientists now believe it is NOT permanent and that regeneration occurs if the immune system is brought under control). The lack of insulin causes an increase of fasting blood glucose (around 70-120 mg/dL in the healthy people)that, when it gets to 180 mg/dL it starts to appear in the urine making it sweet "mellitus" (honey in latin). Glycosuria or glucose in the urine causes the patients to urinate more frequently. Of course, patients also drink more and are always thirsty (polidipsia). These are the first clear symptoms of diabetes, that if not recognized, may take to ketoacidic coma, an extremely dangerous situation. Type 1 is lethal unless treatment with exogenous insulin via injections replaces the missing hormone, or a functional replacement for the destroyed pancreatic beta cells is provided (such as via a pancreas transplant, islet transplant, cell therapy or by treatment and regeneration).

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 as of yet, all children with Type 1 diabetes will grow up to be adults with Type 1 diabetes.

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. Although the cause of Type 1 diabetes is still not fully understood, diet and exercise may help insulin to act more effectively.

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.Fact: date=March 2007 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 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.