The pathogenesis of the various periodontal diseases cannot only be explained on the basis of the relationship of specific bacteria to the clinical features of the disease. The microbiologic examination of various members of the same family, either by culturing or by the use of the DNA probe often showed similarities among several individuals in the same family. This could be accounted for on the basis of bacterial transmission or by contamination. These observations suggest the incidence of familial and genetic factors in the etiology of some forms of periodontitis.