A great amount of clinical and experimental evidence has been accumulated on the role of immunological early events in the development of type 1 diabetes mellitus. In order to try to make a precocious diagnosis of type 1 diabetes mellitus, the Authors have studied all the components of the family in a group of diabetic children. This study is a part of a collaborative International Study called IFS (International Family Study). The Authors describe the scheme of this project in detail. During this study the Authors made a very early diagnosis of diabetes in a child, brother of another diabetic boy. The importance of this approach to the diabetic disease is discussed.