FAMILY CASE STUDIES HELP US IDENTIFY host risk factors in periodontal disease. In this study we examine a family consisting of a mother (40 years old, with rapidly progressive periodontitis), her elder daughter (14 years old, with localized juvenile periodontitis), and younger daughter (13 years old, with simple gingivitis). We examined 1) the peripheral neutrophil functions (chemotactic migration, phagocytosis, superoxide production); 2) lymphocyte functions (proliferative activity and cytokine productivity of T cells, immunoglobulin [Ig] M productivity of B cells when stimulated with pokeweed mitogen); 3) phenotypic analyses of peripheral lymphocyte subpopulations; 4) serum IgG antibody titers against periodontopathic bacteria; and 5) serological type of HLA class IS. All the subjects exhibited high T4/T8 ratios due to high percentage of CD4-positive cells, showed high IgG titers to Actinobacillus actinomycetemcomitans, and had a HLA DQw1 in common. The mother showed a slight deficiency of neutrophil chemotactic migration to N-formyl methyonyl leucyl phenylalanin (fMLP), raised interleukin-2 productivity of T cell, and high levels of IgG titers to Porphyromonus gingivalis and Fusobacterium nucleatum. Both daughters showed weak T cell proliferative response to anti-CD3 monoclonal antibody and low IgM productivity. Low lymphocyte responsiveness may be involved in the pathogenesis of periodontal disease of these daughters; therefore, the lymphocyte dysfunctions shown should be considered in relation to the progression of periodontal disease.