OBJECTIVE: To evaluate whether T helper 1 (Th1) immune response is predominant in women with reproductive failures (recurrent spontaneous abortion and recurrent implantation failure) and the activation of T cell is related to Th1 propensity. METHODS: Women with a history of recurrent implantation failure or recurrent spontaneous abortion comprise the study group (n=37). Controls are normal fertile women without a history of infertility or pregnancy losses (n=11). Th1/Th2 ratios of interferon (INF)-gamma/interleukin (IL)-10 and tumor necrosis factor (TNF)-alpha/IL-10 expression on CD3+/4+ cells, CD154, and CD69 expression on T cells are measured by flow cytometric analysis. RESULTS: The ratios of TNF-alpha to IL-10 expressing on CD3+/4+ cells (Th1/Th2 cell ratios) are significantly higher in study group (42.1+/-2.3) as compared with that of controls (28.7+/-2.7) (p=0.002). The overall trend of CD154 and CD69 expression on T cells are elevated in study group than those of controls. The proportion (%) of CD3+/4+/154+ cells (1.7+/-0.5 vs. 0.3+/-0.2, p=0.038) and the % of CD3+/8+/154+ cells (0.6+/-0.2 vs. 0.1+/-0.0, p=0.024) are significantly higher in study group. The % of CD3+/69+ cells (5.6+/-1.9 vs. 1.3+/-5.4, p=0.046) and % of CD3+/8+/69+ cells (4.8+/-1.3 vs. 1.8+/-0.2, p=0.035) among CD3+/8+ cells are significantly increased in study group. CONCLUSION: Women with reproductive failures have Th1 propensity with increased T cell activation. These finding means that activated T cell has a harmful effect on early pregnancy and implantation by induction of Th1 immunity.