Rapidly progressive periodontitis (RPP) has been suggested as a distinct clinical entity within the spectrum of early onset periodontitis. Immunological mechanisms have been considered in the pathogenesis of RPP. This study was designed to evaluate the distribution and phenotypic properties of the lymphocyte populations within the affected gingival tissue of patients with RPP. Biopsies were obtained from 16 patients between 22 and 33 years of age. The tissue samples were processed for both histopathological and immunohistochemical examinations. Gingival tissue T lymphocytes (CD3+), helper T cells (CD4+), suppressor-cytotoxic T cells (CD8+), and cells positive for HLA-DR antigen were identified using monoclonal antibodies with an immunoperoxidase technique. Intracytoplasmic immunoglobulin-containing cells were also stained immunohistochemically with polyclonal antibodies. CD3+ cells were mainly located beneath the pocket epithelium. CD4+ and CD8+ cells were evenly distributed within this T-cell infiltrate with a CD4+/CD8+ ratio of 1:12. Numerous HLA-DR+ cells were also observed in the lymphocytic infiltrates. The majority of mononuclear cells located throughout the stroma were IgG+ plasma cells. Our results indicate that RPP patients present an IgG-bearing plasma cell dominated lesion with equal participation of both T-cell subpopulations. These findings suggest that activation and proliferation of B-cells play an important role in the pathogenesis of periodontal diseases.