The histopathologic characteristics of the decidual reaction in the uterus of aging mice and the "mesenchymal lesion/tumor" in the urinary bladder of aging mice are compared and found to be very similar. Both lesions consist of spindle and epithelioid cells, may contain round eosinophilic granules and possess nuclear progesterone receptors and cytoplasmic desmin. The decidual reaction derives from endometrial stromal cells, while the "mesenchymal lesion" apparently develops from mesenchymal cells near the trigone area, carrying or developing progesterone receptors. If the hypothesis is accepted that in aging mice the uterine decidual reaction and the "mesenchymal lesion" in the urinary bladder represent an equivalent type of tissue reaction, then it follows that the typical "mesenchymal lesion" is not a tumor and could be called more specifically "decidual-like reaction".