The carboxylic acids are known to be produced by the bacteria in the dental plaque. The purpose of the present study is to investigate the concentrations of carboxylic acids in the whole saliva and the gingival crevicular fluid (GCF) of periodontal patients. The effects of these acids on human gingival fibroblasts were also examined. Nine males and 9 females patients were divided into two groups (Group A and Group B), according to the differences in probing depth (PD) and alveolar bone loss. Patients of Group A had gingivitis or slight adult periodontitis (PD less than or equal to 4 mm, alveolar bone loss less than or equal to 33%), whereas patients of Group B had moderate to advanced adult periodontitis (PD greater than 4 mm, alveolar bone loss greater than 33%). Carboxylic acids in the whole saliva and the GCF which were collected from the patients of Group A and Group B were examined using high performance liquid chromatography. The effects of two carboxylic acid salts (butyrate and formate) on the proliferation of human gingival fibroblasts from normal human gingiva were also examined by cell culture. Results obtained were as follows. 1. The concentrations of four carboxylic acids (acetic, propionic, succinic and butyric acids) in the GCF collected from patients of Group B were significantly higher than that of Group A. On the contrary, all carboxylic acid contents in the whole saliva were not significantly different between the patients of Group A and Group B. 2. Formate (1 or 3 mM) showed only a small degree of inhibition against the proliferation of human gingival fibroblasts, but butyrate (3 mM) almost completely (95.2%) inhibited the proliferation of human gingival fibroblasts. 3. The results of the present study may indicate that the carboxylic acids are involved in the initiation and development of the periodontal disease and that they can be one of the indicators in the diagnosis of the periodontal disease.