The levels of cathepsin G in gingival crevicular fluid (GCF) from chronic adult periodontitis patients and experimental gingivitis subjects were determined both by activity measurement using N‐benzoyl‐(DL)‐phenylalanine‐2‐naphthyl ester as a substrate and by enzyme immunoassay using anti‐human cathepsin G IgG. The activity level of cathepsin G in GCF of both periodontitis and experimental gingivitis has no significant correlation with all measured clinical parameters. Western immunoblotting using antibodies specific for cathepsin G or α1‐proteinase inhibitor revealed that the difficulty in demonstrating the association of its activity with the severity of these diseases was due largely to formation of the enzyme‐inhibitor complexes. By contrast, statistically significant positive correlation was found between cathepsin G content in GCF of periodontitis, which was determined by enzyme immunoassay, and such clinical parameters as the GCF volume, the gingival index and probing depth. The increased cathepsin G content with increasing severity of periodontal inflammation was markedly diminished by the initial treatment. Although no significant activity was detectable in GCF of experimental gingivitis, a rapid increase of the immunoreactive cathepsin G was found in GCF at 3–5 d after refraining from oral hygiene measures, which rapidly decreased by 10 d. The progressively increased cathepsin G between 10th and 21st d rapidly decreased by cleaning of the teeth. The results indicate that cathepsin G is involved in the host's defensive mechanism against the invasion of etiologic microbes and/or the development of either periodont