Histologic studies have been carried out, of fragments from the epiploon obtained from 50 pregnant women. The samples were obtained during cesarean section. In 40% of the patients decidual-type modifications were noted, involving either a limited number of cells, or larger numbers of cells in disseminated plaques. The modification is similar with that noted in the chorion of the endometrium during pregnancy, and we must recall the fact that the peritoneal mesenchyma and the endometrial chorion have the same embryological origin. We believe that this transformation is the result of a modulation process, and that it is completely reversible.