We immunohistologically examined the cell densities and distribution of T and B cells in periodontitis and analyzed their relationship in terms of periodontal tissue destruction. Specimens were labeled with monoclonal antibodies for T cells (CD3) and B cells (CD20) and developed using the double staining technique. CD3^+ cells appeared in low density infiltrates and increased gradually with the number of total infiltrates; in contrast, CD20^+ cells did not appear in small infiltrates but increased rapidly with the number of infiltrates. The number of fibroblastic cells negatively correlated with that of the total infiltrates and CD20^+ cells, but it did not correlate with the number of CD3^+ cells. These findings suggest that the T cells infiltrated into the inflamed sites at the onset and increased gradually with the development of inflammation and that B cells infiltrated later than T cells and their number increased rapidly with inflammation.