Diabetes is a multifactorial disease, the pathogenesis of which involves participation of the host immune system in beta cell destruction. Studies on identical twins offer the opportunity to define genetic and non-genetic factors which may contribute to susceptibility to the disease. Our own work indicates that the number and nature, as well as the intensity and persistence of immune abnormalities of cellular and humoral immune responses can be powerful predictors of the disease and also identify twins who have a degree of protection from the disease.