The enzyme glutamic acid decarboxylase (GAD) is of great importance for the neurotransmission in the central nervous system, and therefore of interest for treatment of pain and neurological disease. However, it is also released in pancreas although its role is not quite clear. GAD is a major auto-antigen in the process leading to type 1 diabetes with both a clear cell-mediated immune response to GAD and auto-antibodies to GAD (GADA), which can be used as a predictor of diabetes. Administration of the isoform GAD65 can prevent autoimmune destruction of pancreatic beta cells in non-obese diabetic (NOD) mice and the subsequent need for exogenous insulin replacement. In Phase I and II studies an alum-formulated vaccine (Diamyd) has shown to be safe, and in a dose-finding study in Latent Autoimmune Diabetes in Adults (LADA) patients 20-microg was given subcutaneously one month apart indicating preservation of residual insulin secretion. A double-blind randomized Phase II trial in 70 patients (10-18 years old) with recent-onset type 1 diabetes showed significant preservation of residual insulin secretion and a GAD-specific immune response, both humoral and cell-mediated, but no treatment-related adverse events. With this promising background further studies are on their way, both intervention in newly diagnosed type 1 diabetic patients, and trials to prevent the disease.