Sex hormones are closely related to the onset and progression of periodontal disease. The purpose of this study was to evaluate the effects of sex hormones on the metabolism of human periodontal ligament cells. Human periodontal ligament cells and gingival fibroblasts were prepared from 7 donors with normal gingiva. 17 beta-estradiol and progesterone were purchased from Sigma Chemical Company. The effects of sex hormones on cell morphology, alkaline phosphatase activity, cell proliferation, DNA synthesis, collagen synthesis and non-collagenous protein synthesis were investigated. The results were as follows: 1. Periodontal ligament cells showed osteoinductive-like cells in the cell morphology and alkaline phosphatase activity. 2. DNA synthesis was stimulated by 17 beta-estradiol and progesterone in the presence of 5% fetal calf serum. 3. Collagen synthesis was inhibited by 17 beta-estradiol and progesterone, while non-collagenous protein synthesis was inhibited by 17 beta-estradiol and low concentrations of progesterone. 4. The addition of serum into culture medium was necessary to induce the basic metabolism of human periodontal ligament cells. It was demonstrated that sex hormones are closely related to the metabolism of human periodontal ligament cells.