Prenatal exposure to diethylstilbestrol (DES) causes a significant increase in the carcinogenic response of the hamster's reproductive tract to subsequent DES treatment. Uteri from DES treated females from untreated- and DES treated mothers (C.D & D.D) have abnormal hyperplasia with characteristic finger-like structures projecting into the lumen of the uteri. Inside these papillae along with the rest of the stroma are cystic glands. We found that these glands had no openings into the uterine lumen and that they "begin" and "end" in the stroma. In addition there are two types of cells lining the cystic gland i.e., pale cells and acidophilic cells. Capillary beds surround the cystic glands. We have named these uterine structures "cystadenomatous papilloma". In addition, we found a spectrum of hyperplastic abnormalities in C.D and D.D uteri and carcinoma in situ in D.D uteri. Similar neoplasms have been described in human pathology. Ultrasound observations have demonstrated that in utero exposure to DES may result in uterine hypoplasia and because it appears to be similar to the changes seen in prenatally DES treated (D.C) hamsters, cross sectional areas of C.C (untreated control), D.C, C.D and D.D uteri were compared. Our results show that later in life uterine hypoplasia also occurs in the 100 days old D.C hamsters and since D.C uteri present hyperplasia with cystic structures, our data support the hypothesis that in utero DES-treated human females may later in life develop benign and malignant lesions in their reproductive tract. Much of these data corresponds to what has been found in humans, and consequently warrants further investigation into the use of Syrian hamster as a model to understand the uterine abnormal morphogenesis in regards to hypoplasia, adenocarcinoma and carcinoma development in the human.