Heterologous tumors of the uterus are commonly designated as Müllerian mixed tumors. Their origin is attributed to dispersed embryogenic isles of Müllerian duct. The always high age of the afflicted woman plus the localization in post-climacteric polyps, speak against this assumption. Five personal observations have motivated the more intensive occupation with the histiogenesis of these cases. This has led to the conclusion that these mixed tissue formations do not come from dispersed cell isles, but that the multipotency of proliferated cells has only appeared in later age in connection with a metaplasia.