Serum IgG antibodies to collagen were investigated by using enzyme linked immunosorbent assay (ELISA) in patients with chronic periodontal disease. Patients with varying forms of periodontal disease including gingivitis, juvenile periodontitis, and adult periodontitis were compared with the normal subjects. The mean serum IgG levels of ELISA antibodies to native type I or III collagen in patients with juvenile periodontitis were significantly higher than those of the normal subjects, but no difference was found between the patients with either gingivitis or adult periodontitis and the normal subjects. In addition, the mean serum IgG levels of ELISA antibodies to denatured type I or III collagen in patients with juvenile or adult periodontitis were significantly higher than those of the normal subjects. These findings suggest that antibodies to native and denatured type I or III collagen may be associated with different forms or severities of periodontal disease, especially advanced periodontal destruction.