Retrograde menstruation represents a plausible explanation for the development of most cases of endometriosis; nevertheless, additional factors must contribute to the development of disease in only 10 to 20% of women. The discriminating factor(s) in determining the development of active endometriosis probably involves a complex array of potentially interactive influences including steroid exposure, immunological disturbances, genetic predisposition, and, perhaps, environmental toxin exposure. Matrix metalloproteinases (MMPs), enzymes that mediate normal tissue turnover including endometrial breakdown at menstruation, appear to be involved in the invasive establishment of the disease. In addition, several MMPs appear to be inappropriately expressed in the endometrium of women with this disease in association with a reduced sensitivity to progesterone. Altered regulation of endometrial MMP expression in response to steroids may represent a mechanism linking the invasive potential of refluxed endometrium to the establishment of this disease only in certain women.