Eleven patients with periodontitis, ten patients with juvenile periodontitis and ten control subjects were studied to compare systemic and local immunologic responses and various other blood parameters. A more intensified immunologic response was seen in juvenile periodontitis as compared to periodontitis or controls as evidenced by: (1) greater number of plasma cells and lymphocytes in biopsy specimens of involved gingiva stained by Harris hematoxylin and eosin; (2) significant decrease in serum C4 levels and slightly elevated serum IgG levels as determined by radial immunodiffusion; (3) marked increase of positive fluorescing cells in biopsy specimens stained with antihuman immunoglobulin conjugate and an increase in complement deposition in the same tissues as determined by immunofluorescence. No significant differences among the two groups of patients and control subjects were observed with respect to complete blood counts, coagulation studies, or blood glucose levels. These findings suggest that the immunologic responses in periodontitis and juvenile periodontitis are qualitatively similar, but that the intensity of the response is greater in juvenile periodontitis.