PURPOSE: We report here, our experience of seven patients with epithelioid sarcomas and their serum CA 125 levels, as well as the results of an in vitro and in vivo study of CA 125 expression in epithelioid sarcoma cells and xenografts using three epithelioid sarcoma cell lines. METHODS: In the clinical study, the serum CA 125 levels of seven epithelioid sarcoma patients were examined at multiple time points. Expression of the MUC16 gene that encodes the CA 125 sequence was examined using RT-PCR methods in three epithelioid sarcoma cell lines, FU-EPS-1, SFT-8606 and NEPS, and the CA 125 protein in each cell lysate was examined by Western blot using anti-CA 125 clone OC125 antibody. The concentration of CA 125 in the conditioned medium of each cell line was also measured. RESULTS: In five of the seven epithelioid sarcoma patients, CA 125 levels reflected regression and progression of their disease. The CA 125 concentrations in the conditioned medium of FU-EPS-1, SFT-8606 and NEPS cells were 259, 252, and 6 U/ml, respectively. Strong expression of MUC16 mRNA was shown in FU-EPS-1 and SFT-8606 cells: correspondingly, a thick band was observed by Western blot analysis in only FU-EPS-1 and SFT-8606 cells. CONCLUSION: We concluded that epithelioid sarcoma cells produce and secrete CA 125 into the blood serum and that the elevation of serum CA 125 correlates with disease progression. Therefore, measuring the serum CA 125 level should provide an useful index for diagnosing and monitoring the course of epithelioid sarcoma.