Diabetes mellitus type I was till recently considered a disease affecting children and young adults. Research in the past several years provided evidence that this type of diabetes is found in all age groups and that the majority develops after the age of 35 years. Diagnostic possibilities of type I diabetes were markedly extended due to the introduction of radioimmunoassay of antibodies against glutamate decarboxylase. This examination has a 82% sensitivity and 100% specificity in diabetes. Diabetes mellitus in advanced age is manifested much less dramatically than in child age. It is found in non-obese subjects and usually its clinical manifestation is gradual. At first the patients are as a rule treated by diet. After several months usually sulphonylurea derivatives are started and only later insulin. If the diagnosis of diabetes mellitus type I is beyond doubt due to autoimmune disease, assessed preferably by antiGAD antibodies, immediate substitution therúpy with insulin is fully indicated. The latter can protect the residual function of B cells of the islets of Langerhans of the pancreas and contribute thus to the prevention of microvascular complications of diabetes mellitus.