The purpose of this study is to investigate the responses of human periodontal ligament (HPDL) cells after activating with IL-1β and TNF-α alone of activating with the combination of estrogen. After 48 hours of activation, the levels of MMP-1 in the medium and NOS-3 expression were examined using western analysis and RT-PCR, respectively. The results indicated that of NOS-3. Interestingly, when cells were activated with estrogen and IL-1β simultaneously, the level of MMP-1 and NOS-3 increased up to 2.3 and 5 folds, respectively, compared with 1.3 and 4.2 folds when cells received IL-1β alone. Similarly, activation with estrogen and TNF-α increased the level of MMP-1 and NOS-3 from 2.2 and 1.8 folds up to 3.2 and 2.5 folds, respectively, in comparing with TNF-α treated alone. The results showed that increasing level of estrogen could enhance the effect of pro-inflammatory factors, which consequently enhanced periodontal tissue destruction.The purpose of this study is to investigate the responses of human periodontal ligament (HPDL) cells after activating with IL-1β and TNF-α alone of activating with the combination of estrogen. After 48 hours of activation, the levels of MMP-1 in the medium and NOS-3 expression were examined using western analysis and RT-PCR, respectively. The results indicated that of NOS-3. Interestingly, when cells were activated with estrogen and IL-1β simultaneously, the level of MMP-1 and NOS-3 increased up to 2.3 and 5 folds, respectively, compared with 1.3 and 4.2 folds when cells received IL-1β alone. Similarly, activation with estrogen and TNF-α increased the level of MMP-1 and NOS-3 from 2.2 and 1.8 folds up to 3.2 and 2.5 folds, respectively, in comparing with TNF-α treated alone. The results showed that increasing level of estrogen could enhance the effect of pro-inflammatory factors, which consequently enhanced periodontal tissue destruction.