We have developed an orthotopic model for human endometrial carcinoma in nude mice. The human serous papillary endometrial carcinoma cell line SPEC-2 was injected into the subcutis (ectopic site) or uterine wall (orthotopic site) of athymic mice. Tumors grew in both locations locally. However, only uterine wall tumors produced metastases in regional and distant lymph nodes and to the lungs and liver. Cell lines were established in culture from these uterine tumors and from lung and liver metastases, and then these cells were injected into the uteri of additional mice. The metastatic potential of the lines subsequently established from tumors growing in vivo was not significantly higher than the already highly metastatic parental culture cells. All SPEC-2 cell lines expressed high levels of both 72-kDa and 92-kDa collagenase type IV activity. mRNA for transforming growth factor-α, basic fibroblast growth factor, and epidermal growth factor-receptor was constant among the cell lines. These data support the concept that the orthotopic implantation of human endometrial carcinoma cells into the uteri of nude mice provides a valuable model for studying the biology of human endometrial adenocarcinoma.