['We aim to determine whether peripheral blood T cell activation is associated with repeated implantation failures or recurrent pregnancy losses (RPLs).', 'Women with a history of repeated implantation failure (n = 18) or RPLs (n = 17) comprise the study group. Normal fertile women (n = 11) are included as controls. Proportion of activated peripheral blood T cells (CD69(+), CD154(+)) and Th1/Th2 cell ratios are measured by flow cytometric analysis.', 'Proportions (%) of CD4(+)/154(+) of CD4(+) and CD8(+)/154(+) of CD8(+) cells were significantly higher in study group than those of controls. Proportions (%) of CD3(+)/69(+) of CD3(+) cells and CD8(+)/69(+) of CD8(+) cells were significantly increased in study group compared to controls. Proportion (%) of CD4(+)/69(+) cells significantly correlated with % CD4(+)/154(+) cells (P = 0.003). Activated cytotoxic T cells (CD8(+)/154(+), CD8(+)/69(+)) inversely correlated with INF-gamma/IL-10 producing CD3(+)/4(+) T cell ratios. Proportion of activated CD3(+)/8(+)/69 and CD3(+)/8(+)/154(+) cells was inversely correlated with IFN-gamma/IL-10 expressing CD3(+)/4(+) T cell ratios.'] Do women with multiple implantation failures and recurrent pregnancy losses have increased peripheral blood T cell activation?