A new human endometrial adenocarcinoma cell line, Watanabe cells, was established from the ascitic fluid of a relapsed endometrial adenocarcinoma obtained from a 58-year-old woman; this cell line has been maintained in vitro for more than 3 years and 8 months. The cells formed a monolayer in a mosaic fashion and tended to pile up and formed a hemicyst. The population boubling time was 60.0 hours at the 10th generation. The modal chromosomal number of the cells was in the diploid range. The histology of tumors induced by this cell line in athymic nude mice showed poorly differentiated adenocarcinoma, while the initial tumor was a well differentiated adenocarcinoma. Estrogen and progesterone receptors (ER, PR) were demonstrated in the original tumor, whereas ER but not PR were present in the tumors induced in nude mice. CA125, CA19-9 and other tumor markers were positive in culture media of this cell line. The cells showed intrinsic cisplatin-resistance (50% inhibition concentration: > 10 micrograms/ml) at 120 hours of exposure by MTT assay. We believe this cell line will be useful for investigating the mechanisms of progesterone therapy, the biological behaviors of the tumor markers and mechanisms of chemotherapeutic resistance in endometrial carcinoma.