The peripheral blood T-cell phenotype of patients with rapidly progressive periodontitis (RPP) was determined in order to investigate whether there were T-cell imbalances or not. Twenty patients aged 21-39 yr were selected for this study. Bone resorption and probing pocket depth were measured. All the patients had 10 or more teeth showing bone loss of 50% or more. As controls, 12 periodontally healthy, age-matched individuals were selected. Blood samples were obtained by venipuncture, and lymphocytes were isolated. Two-colour flow cytometric analysis was done with monoclonal antibodies against human CD4, CD8, and CD45RA antigens. The RPP patients were found to have significantly lower percentage of CD8+ T cells (Mann-Whitney's U-test, p < 0.01) and an increased CD4/CD8 ratio (Mann-Whitney's U-test, p < 0.01) compared with healthy individuals. On the other hand, there were no significant differences in the percentages of CD4+ T cells and CD4+ CD45RA+ T cells between RPP patients and healthy individuals. No correlations between the clinical findings and T-cell subsets were found. These findings suggest that imbalances of peripheral blood T lymphocytes, especially a tendency to decreased CD8 + T cells, exist in RPP patients, and that cellular immune responses mediated by CD8 + T cells may play a part in the pathogenesis of RPP.