Purpose: The purpose of this study is to identify a prospective association between CA125 and tumorigenic ovarian cancer cells, using the new method of orthotopic transplantation (1). Method: After making the surgical ovarian cancer specimen into cell suspension, we separated the tumorigenic cells from the nontumorigenic cancer cells based on cell surface marker (cancer antigen CA125 and lineage markers) expression. We developed a SCID mice model in which the CA125+/ lineage- and CA125-/ lineage- cells were injected into ovarian parenchyma by use of a microinjector. As a measure of effectiveness of tumor-forming, tumor weight, abdominal distension, ascites volume and activity, subcutaneous fat were determined or observed. Immunohistochemistry was done to determine tumor cell markers. Results: We found that the cells of CA125+/ lineage- were able to form new tumors; whereas, an equal quantity of CA125-/ lineage- cells failed to form any tumors. The new generated tumor contained additional CA125-/ lineage-tumorigenic cells as well as the phenotypically diverse population of nontumorigenic cells. Quantities were judged to be significantly different P < 0.0001. Conclusion: CA125+/ lineage- cells, which may be ovarian cancer stem cells, were the source for tumor recurrence. The strategies designed to target this cell population may lead to more effective therapies.