We report 2 cases of primary vaginal mucinous adenocarcinoma arising in adenosis in nondiethylstilbestrol-exposed women, 1 with uterus didelphys. Both tumors exhibited morphologic and immunohistochemical features (MUC6 and/or HIK 1083 positivity) identical to the recently described cervical gastric-type adenocarcinoma, a subtype of mucinous adenocarcinoma that is non-human papillomavirus related and possibly related to adenoma malignum. Both neoplasms were intensely p53 positive, suggesting that TP53 mutation may be implicated in their development. We believe that the vaginal tumors arose from adenosis through atypical adenosis, as benign and atypical glands were present at the periphery of the neoplasms. In reporting these cases, we discuss atypical adenosis and other types of non-diethylstilbestrol-associated vaginal adenocarcinomas. At least 9 other examples of primary vaginal, or more uncommonly cervical, adenocarcinomas arising in non-diethylstilbestrol-exposed women with congenital genitourinary malformations have been reported, suggesting a probable causal association between congenital malformation, vaginal adenosis, and vaginal adenocarcinoma.