The purpose of this study was to investigate the differences of histochemical characteristics in inflammatory fibrous gingival hyperplasia (FGH), phenytoin-induced gingival hyperplasia(PIGH), idiopathic gingival hyperplasia(IDGH) and control groups (healthy and inflammatory gingiva) by immunohistochemical method with various antibodies and histomorphological analysis. In immunohistochemical finding, antibodies to inflammatory cells (T/B lymphocytes, macrophages, other monocytes), proliferating cell nuclear antigen(PCNA), epidermal growth factor(EGF), factor VIII, and type I collagen were used. 1. The inflammatory infiltrates in FGH were less than those in inflammatory gingiva. The composition of inflammatory cells of PIGH was similar with that of FGH. IDGH showed a similar histologic findings with healthy gingival tissue. 2. In FGH, the number of fibroblasts and newly-formed collagen fibers was increased. No significant increase of fibroblasts and the dense accumulation of thick collagen fibers were seen in PIGH. The increase of fibroblasts and the dense accumulation of thick collagen were seen in IDGH. 3. PCNA-positive cells were localized mainly in the area accumulated with inflammatory cells and blood vessels, significantly increased in all hyperplastic tissue groups, and distributed evenly in IDGH. 4. The distribution of EGF were not observed in healthy gingiva but detected locally in area with confluent blood vessels,without significant difference between the other tissue groups. This results suggest that inflammation plays a significant role in inducing hyperplastic change of gingival tissue. While in DIGH, drug itself as well as inflammation seems to attribute to hyperplastic change.