We report embryo-induced alterations occurring in endometrial stromal cells (ESCs) during the embryo-attachment stage in bonnet monkeys (Macaca radiata). Laser micro-dissected ESCs obtained from pregnant and non-pregnant animals were compared for levels of selected proliferation and decidualization-associated factors by analysis with quantitative real-time polymerase chain reaction or immunohistochemistry. Stromal cells exhibited extensive cellular proliferation, as indicated by cellular compaction and significantly higher (P < 0.05) levels of proliferating cell nuclear antigen and of estrogen receptor 1, c-Myc, and Cyclin D1 transcripts in pregnant animals as compared with non-pregnant animals. A significant decrease (P < 0.05) was observed in the transcript levels of stromal interleukin-6 (IL-6) in pregnant animals. Cell proliferation was accompanied by a significant increase (P < 0.001) in the levels of decidualization-associated molecules such as IL-1β in the luminal and glandular epithelium and of stromal insulin-like growth-factor-binding protein-1 (IGFBP-1) and prostaglandin-endoperoxide synthase-2 (PTGS-2) proteins. In pregnant animals, proliferation was evident throughout the gestational stroma, whereas decidualization was more pronounced in the embryo-attachment zone than in the non-attachment zone. To our knowledge, this is the first report of alterations in the endometrial stroma during the embryo-attachment stage in a non-human primate model.