Notes: Background:  It is important to clarify the biological function of the female sex hormones estrogen and progesterone in periodontal ligament cells, as these hormones may affect periodontal health. We have previously shown that human periodontal ligament cells express estrogen receptor β (ERβ) but not ERα, whereas human breast cancer cells (MCF7) express both ERα and ERβ. Data on progesterone receptor (PgR) expression in human periodontal ligament cells have not been reported.Objectives:  Determine PgR expression in human periodontal ligament and MCF7 cells and to investigate how estrogen affects DNA and collagen synthesis in these two cell types showing different pattern of expression for ERα and β.Methods:  Periodontal ligament cells were obtained from the periodontal ligament of premolars extracted for orthodontic reasons and MCF7 cells from the American Type Culture Collection (ATCC). PgR expression was determined by immunocytochemistry. DNA and collagen synthesis was determined by [3H]thymidine and l-[3H]proline incorporation, respectively.Results:  PgR immunoreactivity was observed in nuclei of MCF7 but not periodontal ligament cells. Treatment with estrogen (17β-estradiol, E2) at physiological concentrations for 24 h stimulated DNA synthesis by more than two times in MCF7 cells, whereas there was no effect on periodontal ligament cell DNA synthesis. The ER blocker ICI 182780 fully reversed the stimulatory effect of E2. Not only short-term (24 h) but also long-term (5 days) treatment with E2 lacked effect on DNA synthesis in periodontal ligament cells. Neither periodontal ligament cell viability nor collagen synthesis was affected by E2 treatment. Identical results were observed in periodontal ligament cells from male and female subjects.Conclusions:  Human MCF7 but not periodontal ligament cells express PgR, suggesting that progesterone via PgR affects MCF7 but not periodontal ligament cells. Further, estrogen stimulates breast cancer MCF7 cell proliferation, whereas it has no effect on proliferation of periodontal ligament cells, probably reflecting cell type specific ER expression pattern in these two cell types.