Human thioredoxin, a cellular disulphide reducing protein, is known to be secreted by some types of cells and to display unique extracellular activities including modulation of cytokine actions and protection of the cell against damage from oxidative stress. This study has been undertaken to investigate the pattern of expression and tissue distribution of thioredoxin in human endometrium during the menstrual cycle. Immunohistochemical studies showed increased thioredoxin immunoreactivity in the glands of the secretory phase compared to those of the proliferative phase. Although the staining of thioredoxin was relatively intense in predecidual stromal cells, the most prominent staining of thioredoxin was present in both glands and stroma of the endometrium in the early secretory phase of the menstrual cycle. Northern hybridization analyses revealed that expression of thioredoxin mRNA in the endometrium of the early secretory phase was approximately 3-fold compared to the other phases of the menstrual cycle, consistent with the results of the immunohistochemical studies. These results suggest that both protein and gene expression of thioredoxin in the endometrium are menstrual cycle phase-specific and highly active in the phase of endometrial differentiation which occurs in preparation for implantation (early secretory phase of the menstrual cycle). Thioredoxin expressed in the early secretory phase of the menstrual cycle may be advantageous for blastocyst implantation.