A case of adenocarcinoma developing in endometrial and adenomyotic epithelium is presented. The pattern of development of the carcinoma is discussed in relation to the histogenesis of adenomyosis. The authors conclude that the malignant transformation occurred in a susceptible field that included both endometrial and adenomyotic epithelium. The rare examples of adenocarcinoma arising in and confined solely to adenomyosis represent limit cases. The possible effect of estrogen is reviewed.