Cell lines are very useful for clinical and basic research. The establishment of uterine malignant tumor cell lines with unusual histology is especially important. We describe the establishment and characterization of a new human endometrial stromal sarcoma cell line of the uterus. The cell line OMC-9 was established from a tumor mass in the uterine body of a 55-year-old woman. Characteristics of the cell line studied include morphology, chromosome analysis, heterotransplantation, tumor markers and chemosensitivity. This cell line has grown well for 196 months and has been subcultured more than 50 times. Monolayer cultured cells are polygonal in shape, appear to be spindle-shaped or multipolar and have a tendency to pile up without contact inhibition. The cells exhibit a human karyotype with a modal chromosomal number in the diploid range. The cells were able to be transplanted into the subcutis of nude mice and produced tumors resembling the original tumor. OMC-9 cells produced tissue polypeptide antigen. Both CD10, a sensitive and diagnostically useful marker of endometrial stromal neoplasms, and vimentin were identified immunohistochemically in the original tumor and the heterotransplanted tumor. The cells were sensitive to actinomycin D, doxorubicin, carboplatin, cisplatin and etoposide, drugs used commonly in the treatment of gynecologic cancer. Only three reports of uterine endometrial stromal sarcoma cell lines have thus far been reported in the literature. OMC-9 is the first endometrial stromal sarcoma cell line in which CD10 expression and chemosensitivity have been identified.