The logical clinical approach to diagnosis is history, physical examination, and laboratory studies, in the order given. For didactic purposes, however, it is advantageous to begin the discussion of the diagnosis of the hereditary hemorrhagic diseases with a consideration of laboratory studies because all known congenital bleeding diseases, with the exception of telangiectasia, appear to have a defect in the clotting mechanism which can be detected by laboratory tests. The first requisite, therefore, for the diagnosis of this group of diseases is an understanding of the principal reactions involved in the clotting process. A clotting scheme has