Remodelling of endometrial tissues is fundamental to the cyclical changes that occur during the menstrual cycle, implantation and, in the absence of pregnancy, at menstruation. The enzyme matrix metalloproteinase-9 (MMP-9) is recognized as important in these processes but its regulation is not well defined. These studies have demonstrated that MMP-9 activity is present in the endometrium and exhibits cyclical changes in its distribution in the glandular and stromal cells. MMP-9 protein is present throughout the cycle with highest expression, as determined by semiquantitative analysis of specific MMP-9 immunoreactivity, in glandular cells during the mid secretory phase. A similar distribution was observed in first trimester decidua. In women with a levonorgestrel intrauterine system (LNG-IUS), which delivers high local concentrations of progestagen to the uterine cavity, MMP-9 is highly expressed in both endometrial glandular and stromal cells, and in the vasculature (in endothelial and perivascular cells). It can be concluded that MMP-9 is stimulated directly or indirectly by progesterone. Furthermore, MMP-9 may play a role in the remodelling of the endometrium that occurs during the menstrual cycle and in the aetiology of the morphological changes and breakthrough bleeding associated with long-term progestagen administration via a LNG-IUS.